RAWALPINDI, Pakistan (CNN) -- Pakistan's former Prime Minister Benazir Bhutto was assassinated Thursday after addressing a large gathering of her supporters.

The bomb explodes near Bhutto's vehicle following a political rally in Rawalpindi.

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The suicide bomb attack also killed at least 22 others, doctors said. It was not immediately clear if Bhutto died from shots fired before the blast, or from wounds caused by bomb shrapnel.

Her body was removed from Rawalpindi General Hospital late Thursday night, about six hours after the assassination.

President Pervez Musharraf held an emergency meeting in the hours after the death, according to state media.

He said the killers were the same extremists that Pakistan is fighting a war against, and announced three days of national mourning.

Video of the scene just moments before the explosion showed Bhutto stepping into a heavily guarded vehicle to leave the rally.

John Moore, a photographer for Getty Images, said he heard at least two gunshots before the bomb was detonated.

Police sources told CNN the bomber, who was riding a motorcycle, blew himself up near Bhutto's vehicle. Watch aftermath of the attack. »

Bhutto was rushed to Rawalpindi General Hospital -- less than two miles from the bombing scene -- where doctors pronounced her dead.

Chaos erupted at the hospital when former Prime Minister Nawaz Sharif arrived to pay his respects to Bhutto less than three hours after her death.

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