Bomb kills 45 at Pakistan parade

• NEW: Two groups claim responsibility for bombing, and one promises more attacks


• Footage shows onlookers crying as authorities rush the injured to ambulances


• 5 kilograms of explosives used, police say; suicide bomber's body parts found


• Pakistan and India take part in daily "lowering of the flags" ceremony at border

(CNN) -- At least 45 people were killed and 55 more injured in a Lahore, Pakistan, suicide blast near the country's border with India, police and hospital officials said Sunday.

A suicide bomber detonated his vest after a daily parade at the Wagah border crossing, Inspector General Mushtaq Ahmad Sukhera of the Punjab police said. It appeared the bomber entered the parade area as people were leaving and set off an explosive vest near a checkpoint, he said.

Spokesmen for two groups linked to the Pakistani Taliban -- Jundallah and Jamaat-ul-Ahrar -- separately claimed responsibility for the bombing, with the spokesman for the latter, Ehsanullah Ehsan, calling the Jundallah claim "baseless." Ehsan's statement promised more attacks.

"We will soon release the video of this attack," the statement said. "This attack is the revenge of the killing of those innocent people who have been killed by Pakistan Army, particularly of those who have been killed in North Waziristan."

Footage from CNN affiliate GEO News showed authorities scrambling through the streets, some of them ferrying the injured on stretchers to ambulances. Many onlookers were crying as they spoke to reporters.

Police have cordoned off the area and are investigating, Sukhera said.

Sukhera and a doctor in the emergency ward at Ghurki Trust Teaching Hospital said the death toll was 45 people, according to the latest count.

The death toll has risen rapidly since the explosion was first reported.

More than 5 kilograms (11 pounds) of explosives were used in the blast, which was conducted by a male from 18 and 20 years old, the inspector general said.

Some of the bomber's body parts were found at the scene, he said.

Indian Prime Minister Narendra Modi sent his condolences to the families of the deceased, he tweeted shortly after condemning "the dastardly act of terrorism."

Authorities were aware of threats of terrorism, and Pakistan's paramilitary security force, the Rangers, was prepared, but the crowds at the border were larger than normal so Rangers weren't able to check everyone as they departed the area, Sukhera said.

Every evening for 55 years at the Wagah border crossing between Lahore and Amritsar, India, Pakistan's Rangers and India's Border Security Force take part in a lowering-of-the-flags ceremony before sunset.

The ceremony, which ends with soldiers folding their countries' flags and shaking hands, draws attendees from both sides of the border, as well as international tourists.

The "beating retreat" ceremony, as it's known, is postponed until Thursday, officials said.

CNN's Eliott C. McLaughlin and Anna-Maja Rappard contributed to this report.