Civic Party member Ken Tsang is taken away by officers, before being allegedly beaten up outside the central government offices in Hong Kong on October 15, 2014. 
Police and pro-democracy protesters scuffle on October 15. Demonstrators are angry about China's decision to allow only Beijing-vetted candidates to run in the city's elections for chief executive in 2017. 
Hong Kong police aim pepper spray at a pro-democracy protester during clashes in the morning of October 15. 
Police advance on a line of pro-democracy protesters standing on a key road near government headquarters on October 14. 
Police forces march toward pro-democracy protesters on a key road outside the central government offices in Hong Kong on October 14. 
Police remove bamboo that pro-democracy protesters had set up to block off main roads in Hong Kong on Tuesday, October 14. 
Pro-democracy protesters watch as police remove barricades in Hong Kong on October 14. 
Cleaners sweep the main road after the police's removal of barricades on October 14. 
Police ask a protester to leave the main road of Hong Kong's Central district on October 14. 
Police dismantle barricades from the streets in Hong Kong on October 14. 
Police officers run to barricades set up by protesters on October 14. 
Pro-democracy protesters raise their hands behind police officers after people tried to remove the metal barricades that protesters set up to block off main roads near the city's financial district in Hong Kong on Monday, October 13. 
A police officer tries to stop a man October 13 from removing metal barricades set up by protesters. 
A police officer scuffles with a man in Hong Kong on October 13. 
Police officers arrest a pro-democracy demonstrator in the Admiralty district of Hong Kong on October 13. 
A pro-democracy demonstrator sets up a new barricade made of bamboo in Hong Kong on October 13. 
Police officers remove barricades used by protesters in Hong Kong on October 13. 
Taxi drivers protest in Hong Kong on October 13, urging pro-democracy demonstrators to clear the roads. 
People gather beneath the statue "Umbrella Man," by the Hong Kong artist known as Milk, which has become a symbol at the protest site, on Saturday, October 11, in Hong Kong. 
Pro-democracy protesters remain scattered at the protest site in Admiralty on Thursday, October 9. The government canceled talks that day after protest leaders urged supporters to keep up the occupation. 
Taxi drivers attend a small demonstration calling for protesters to stop blocking roads through the city on October 9. 
A woman drinks a soda as she walks past a barricade erected by pro-democracy demonstrators in Hong Kong on October 9. 
A pro-democracy protester sleeps on a street in the occupied area surrounding the government complex in Hong Kong on Wednesday, October 8. 
A pro-democracy protester reads a newspaper in Hong Kong's Mong Kok district on Tuesday, October 7, as a police officer stands nearby. 
Protesters walk up an empty street inside the protest site near Hong Kong's government complex on October 7. 
Joshua Wong, a 17-year-old student protest leader, is interviewed at the protest site near government headquarters on Monday, October 6. 
A man walks to work as pro-democracy demonstrators sleep on the road in the occupied areas surrounding the government complex in Hong Kong on October 6. 
A ray of sunlight bathes sleeping protesters as they occupy a major highway in Hong Kong on October 6. Protesters say Beijing has gone back on its pledge to allow universal suffrage in Hong Kong, which was promised "a high degree of autonomy" when it was handed back to China by Britain in 1997. 
People take an escalator to work as protesters sleep on October 6. 
The statue "Umbrella Man," by the Hong Kong artist known as Milk, stands at a pro-democracy protest site in the Admiralty district on October 6. 
People walk to work on a main road in the occupied areas of Hong Kong on October 6. 
Police officers remove barriers outside government offices in Hong Kong on Sunday, October 5. 
Student protesters carry a barrier to block a street leading to the protest site on October 5. 
Pro-democracy demonstrators occupy the streets near government headquarters on October 5. 
Pro-democracy demonstrators surround police October 5 in the Mong Kok district of Hong Kong. 
A pro-democracy protester holds on to a barrier as he and others defend a barricade from attacks by rival protest groups in the Mong Kok district on Saturday, October 4. 
Pro-democracy student protesters pin a man to the ground after an assault during a scuffle with local residents in Mong Kok on October 4. 
Pro-democracy protesters raise their arms in a sign of nonviolence as they protect a barricade from rival protest groups in the Mong Kok district on October 4. 
A pro-Beijing activist holds up blue ribbons for anti-Occupy Central protestors to collect as pro-government speeches are made in the Kowloon district of Hong Kong on October 4. 
A man sits in front of a barricade built by pro-democracy protesters on October 4 in the Kowloon district. 
Thousands of pro-democracy activists attend a rally on the streets near government headquarters on October 4 in Hong Kong. 
A group of men in masks fight with a man who tried to stop them from removing barricades from a pro-democracy protest area in the Causeway Bay district of Hong Kong on Friday, October 3. 
Police raise hands against protesters as an ambulance tries to leave the compound of the chief executive office in Hong Kong on October 3. 
A protester tries to negotiate with angry residents trying to remove barricades blocking streets in Hong Kong's Causeway Bay on October 3. Large crowds opposed to the pro-democracy movement gathered to clear the area. 
Pro-democracy demonstrators protect a barricade from "anti-Occupy" crowds in Hong Kong on October 3. 
A man shouts at a pro-democracy demonstrator on October 3. 
Police try to pry a man from a fence guarded by pro-democracy demonstrators on October 3. 
Pro-democracy demonstrators sleep on the street outside a government complex in Hong Kong on Thursday, October 2. 
As the sun rises, a protester reads during a sit-in blocking the entrance to the chief executive's office on October 2. 
Yellow ribbons, a symbol of the protests in Hong Kong, are tied to a fence as police and security officers stand guard at the government headquarters on October 2. 
Protesters confront police outside the government complex in Hong Kong on October 2. 
Protesters camp out in a street in Hong Kong on Wednesday, October 1. 
Founder of the student pro-democracy group Scholarism, Joshua Wong, center, stands in silent protest with supporters at the flag-raising ceremony at Golden Bauhinia Square in Hong Kong on October 1. 
Hong Kong's Chief Executive C.Y. Leung attends a flag raising ceremony to mark the 65th anniversary of the founding of Communist China on October 1. 
A pro-democracy activist shouts slogans on a street near the government headquarters on Wednesday, October 1. 
Hong Kong police stand guard outside the flag-raising ceremony October 1. 
Pro-democracy demonstrators gather for a third night in Hong Kong on Tuesday, September 30. 
Protesters sing songs and wave their cell phones in the air after a massive thunderstorm passed over the Hong Kong Government Complex on September 30. 
Protesters take part in a rally on a street outside the Hong Kong Government Complex on September 30. 
Student activists rest on a road in Hong Kong on September 30, near the government headquarters where pro-democracy activists have gathered. 
A pro-democracy demonstrator guards a bus covered with messages of support in Hong Kong on September 30. 
Protesters sleep on the streets outside the Hong Kong Government Complex at sunrise on September 30. 
Protesters hold up their cell phones in a display of solidarity during a protest outside the Legislative Council headquarters in Hong Kong on Monday, September 29. 
Protesters put on goggles and wrap themselves in plastic on September 29 after hearing a rumor that police were coming with tear gas. 
Police officers stand off with protesters next to the Hong Kong police headquarters on September 29. 
A man helps protesters use a makeshift ladder to climb over concrete street barricades on September 29. 
Riot police fire tear gas on student protesters occupying streets around government buildings in Hong Kong on September 29. 
Police officers rest after protests on September 29. 
Pro-democracy protesters argue with a man, left, who opposes the occupation of Nathan Road in Hong Kong on September 29. 
Pro-democracy protesters sit in a road as they face off with local police on September 29. 
Pro-democracy protesters rest around empty buses as they block Nathan Road in Hong Kong on September 29. Multiple bus routes have been suspended or diverted. 
Police walk down a stairwell as demonstrators gather outside government buildings in Hong Kong on September 29. 
Stacks of umbrellas are ready for protesters to use as shields against pepper spray on September 29. 
Protesters turn the Chinese flag upside-down on September 29 outside a commercial building near the main Occupy Central protest area in Hong Kong. 
Protesters occupy a main road in the Central district of Hong Kong after riot police used tear gas against them on Sunday, September 28. 
Demonstrators disperse as tear gas is fired during a protest on September 28. There is an "optimal amount of police officers dispersed" around the scene, a Hong Kong police representative said. 
Police use pepper spray and tear gas against demonstrators September 28. The protests, which have seen thousands of students in their teens and 20s take to the streets, swelled in size over the weekend. 
Riot police clash with protesters on September 28. 
Police and protesters clash during a tense standoff with thousands of student demonstrators, recently joined by the like-minded Occupy Central movement, on September 28. 
Benny Tai, center, founder of the Occupy Central movement, raises a fist after announcing the group would join the students during a demonstration outside government headquarters in Hong Kong on September 28. 
Pro-democracy activist and former legislator Martin Lee wears goggles and a mask to protect against pepper spray on September 28. 
A pro-democracy activist shouts at police officers behind a fence with yellow ribbons on September 28. 
A sign for the Hong Kong central government offices has been crossed out with red tape by democracy activists on September 28. 
Pro-democracy protesters gather near government headquarters on September 29. 
Protesters gather during a demonstration outside the headquarters of the Legislative Counsel on September 28 as calls for Beijing to grant the city universal suffrage grow louder and more fractious. 
Protesters tie up barricades on September 28 during a demonstration outside the headquarters of the Legislative Council in Hong Kong. 
An injured protester is tended to after clashing with riot police outside Hong Kong government complex on Saturday, September 27. 
Riot police use pepper spray on pro-democracy activists who forced their way into the Hong Kong government headquarters during a demonstration on September 27. 
People watch from on high as pro-democracy demonstrators are surrounded by police after storming a courtyard outside Hong Kong's legislative headquarters on Friday, September 26. 
Students march to Government House in Hong Kong on Thursday, September 25.
- NEW: Hong Kong authorities promise "impartial investigation" into alleged assault video
- NEW: Clip shows protester being carried to dark corner, kicked and punched
- Police clashed with protesters on Tuesday night as they tried to re-occupy main road
- Pro-democracy protesters have blocked key streets for more than two weeks
Hong Kong (CNN) -- Hong Kong authorities have vowed to conduct an impartial investigation into a video which appears to show several plainclothes police officers beating up a detained pro-democracy demonstrator.
The widely circulated video shows officers carrying the man to a dark corner, where he's laid on the ground and repeatedly kicked and punched while others stand around, apparently keeping watch.
"Police express concern over the video clip showing several plainclothes officers who are suspected of using excessive force," said a statement issued by the Hong Kong Information Services Department Wednesday morning.
"The Complaints Against Police Office has already received a relevant complaint and will handle it in accordance with the established procedures in a just and impartial manner."
Hong Kong's Secretary of Security Lai Tung Kwok said the "concerned personnel will be transferred out of their current positions for the time being." He declined to take questions.
Claims of 'brutal violence'
Photos show bruising on Ken Tsang's face.
The victim has been identified as Ken Tsang, a member of the Civic Party, who's also a social worker and a member of the 1,200 member election committee that choses Hong Kong's leader.
Civic Party leader Alan Leong told CNN: "This is an apparent abuse of police power that a society as civilized as Hong Kong would definitely not swallow. I would advise the commissioner of police to immediately arrest the six officers involved in that attack."
One of protest groups -- Occupy Central With Love and Peace -- issued a statement accusing police of tying Tsang's hands with plastic cable ties and handcuffs before treating him with "brutal violence." They circulated photos showing large circular welts on Tsang's back, bruising on his face and scratches on his neck and arms.
Tsang's lawyer Dennis Kwok said his client was arrested for illegal assembly and obstructing a police officer on duty. He remains in police custody and, under Hong Kong law, can be detained for 48 hours without charge.
Amnesty International called for the prosecution of any officers found to have acted unlawfully. "It is stomach-churning to think there are Hong Kong police officers that feel they are above the law," said Mabel Au, the group's Hong Kong director.
Protest clashes
Police, protesters clash violently in HK 
Police tear down Hong Kong barricades 
Taxi drivers clash with HK protesters 
HK leader faces pressure for payments 
Beijing using 'anaconda' strategy in HK The alleged attack happened in the early hours of Wednesday morning, hours after clashes between police and protesters who had been trying to re-occupy a major road outside the government headquarters.
Pro-democracy protesters repelled the first police attempt to move them on, encircling officers who fired pepper spray before retreating from Lung Wo Road, in the Admiralty district, the epicenter of the weeks long standoff between protesters and police.
"Police returned later in the evening and successfully cleared the protesters and their barricades," reported CNN's Pamela Boykoff.
A total of 45 people -- 37 men and eight women -- were arrested for alleged unlawful assembly and obstructing police officers. In a statement issued on Wednesday, the Hong Kong government said protesters had "behaved in a disorderly manner," throwing traffic cones, snatching police barriers and building road blocks. Four police had been injured in the operation, it said.
During Tuesday night's attempted takeover of Lung Wo road, demonstrators declared they were "not afraid of anything."
"Ask them if they're afraid of bullets. They're not. This is our last chance. We know we're most likely going to fail, but we have to try," said 62-year-old demonstrator Lo Cheong.
Retaking Lung Wo road was necessary "to protect the protesters," said a demonstrator named Bon.
In a statement released on Wednesday morning, police appealed to protesters in the area to "stay calm and restrained." "Protestors advancing against Police cordon line even with their arms raised is not a peaceful act," the statement said.
By dawn, morning rush-hour traffic was flowing as normal. The contingent of demonstrators relocated to a park near the government offices, to a green space that they're occupying with tents.
It was the latest in a series of clashes between protesters and police, and followed a police operation earlier in the day to clear barricades from Queensway, a road adjacent to the main Admiralty protest site.
Dozens of officers cut through plastic ties strapping together metal fences, and hauled away signs, wooden pallets and recycling boxes which had been stacked to block access to the site.
Since late September, protesters have been blocking major roads in an attempt to force the government to give residents a greater say in who leads the Hong Kong Special Administrative Region of China.
Who's who in the protests?
Protest disruption
Hong Kongers grow weary of sit-ins 
Art bursts from Hong Kong protests 
Generational gap in Hong Kong protests 
Protests divide Hong Kong families At the peak of the protests, tens of thousands of demonstrators crowded onto the streets demanding the right to be able to select candidates to run for chief executive in the 2017 election.
Protesters have been guarding barricades erected at protest sites at Admiralty, Mong Kok and Causeway Bay, and for many nights slept in the open air on the asphalt before the arrival of reinforcements with tents over the weekend.
Traffic in the other parts of the city has been clogged due to road closures, bus and tram cancellations and the need for cars to drive around the protest sites. Taxi drivers say their takings are down, and businesses claim the protests have cost them income.
Hope that the two sides might be able to solve the impasse evaporated on Thursday when the government canceled talks planned with student groups, after protest leaders urged supporters to keep up the occupation.
First live address
Last weekend, Hong Kong Chief Executive C.Y. Leung made his first live address since the start of protests in late September.
Who is C.Y. Leung?
Speaking on local free-to-air station TVB, Leung said the protests were not a "revolution," but a "mass movement that has spun out of control."
He said student leaders had "almost zero chance" of pushing Beijing to change its stance on how Hong Kong's leader is elected. He added he would not submit to the protesters' demands that he resign, because his resignation "will not solve the problem."
"It is because the students and other occupation protesters demand more than that. They want the Standing Committee to withdraw its August 31 decision. That is impossible," he said.
Beijing white paper
He was referring to the white paper issued by the Standing Committee of China's National People's Congress stating that Hongkongers would be able to vote on their leader in 2017, but only from an approved shortlist of candidates.
In response to Leung's comments, the three main groups leading the occupy campaign said it was the government that was out of control -- "a government that fires tear-gas at unarmed citizens and unilaterally terminated dialogue with the students (sic)."
On September 28, police fired 87 tear gas rounds into the crowd after protesters failed to disperse. The move was seen as a miscalculation and only served to garner support for the protesters, who accused the government of heavy handedness and of stifling free speech.
The end of police trust?
Open letter to China
Last weekend, student leaders from the protest group Scholarism and the Hong Kong Federation of Students wrote an open letter to Chinese President Xi Jinping, urging him to closely consider their cause.
"There will only be more citizens, disillusioned with our corrupted institutions, marching and protesting, as long as no genuine democracy is practiced in this place," they wrote.
The letter said the occupation was "definitely not a colour revolution or its alike, but rather a movement for democracy," referring to the term "umbrella revolution" which was coined after protesters used umbrellas to shield themselves from pepper spray and tear gas.
Xi has not commented on the protests, but Chinese censors have been busy blocking reports of the movement, including access to the photo-sharing site Instagram.
CNN's Pam Boykoff, Wilfred Chan, Anjali Tsui, Vivian Kam, Elizabeth Joseph, Chieu Luu and Michael Martinez contributed to this report.
