Pakistani protests turn deadly

• NEW: Three people have died in the violent unrest since Saturday


• NEW: News crews from several lslamabad stations have been attacked by police


• About 450 people were injured in clashes, health officials say


• Protesters have taken to the streets in Islamabad for two weeks

Islamabad, Pakistan (CNN) -- Pakistani protesters flooded onto Constitution Avenue in Islamabad for another day of demonstrations Sunday, demanding a solution to the growing political crisis wracking the country.

At least three people have died in the fighting so far, according to Dr. Ayesha Isani, the spokeswoman for the Pakistan Institute of Medical Sciences in Islamabad.

Local news crews, including cameramen from three different channels, were attacked by police trying to disperse protesters Sunday.

Prime Minister Nawaz Sharif is vowing to remain on the job, despite the violent demonstrations against his government.

Security is tight on Constitution Avenue, Pakistan's main political artery, where the nation's major state institutions, including Parliament and the Supreme Court, are located.

Protesters again threatened to march on the prime minister's house in Islamabad, as they did Saturday.

Sharif has failed to negotiate a solution with the protesters and last week asked the military for help brokering an end to the crisis.

About 450 people have been injured in clashes, according to the Pakistan Institute of Medical Sciences and Polytechnic Hospital.

Pakistan's defense minister defended the use of tear gas.

"Tear gas is a normal practice undertaken all over the world to disperse a crowd. It is something that is an alternative to using force," Minister Khawaja Asif said. "The situation had precipitated to a point that if action had not been taken then it would have been a free fall for the government."

At least 8,000 people have rallied in the city's center after allegations of vote-rigging during last year's election. Negotiations between Sharif's government and his opponents, some of whom are calling for his resignation, have reached an impasse.

Sharif has said he will not resign -- a demand he has called "unconstitutional."

Imran Khan -- an enigmatic former cricket star who leads one of Pakistan's largest political parties -- has led the two-week protests.

Khan is demanding new elections, while outspoken cleric Tahir ul-Qadri -- who wants to overhaul the country's political system -- is demanding much more sweeping reforms.

"The leaders of this country should forget that we will ever back down," Qadri told the media Sunday.

As the protests have grown in size and scope, the army has stationed personnel at government buildings in case protesters try to occupy them.

In Karachi, the scene was stable but tense Sunday as the Muttahida Quami Movement party called for a day of mourning. Shops were closed, and there was little activity on the streets of Pakistan's largest city.

Sophia Saifi reported from Islamabad and Holly Yan wrote from Atlanta. CNN's Joshua Berlinger and journalist Adeel Raja contributed to this report.