Bhutto's supporters erupted in anger and grief after her death, attacking police and burning tires and election campaign posters in several cities. At the hospital where she died, some smashed glass and wailed, chanting slogans against President Pervez Musharraf.
The death of the charismatic 54-year-old former prime minister threw the campaign for the Jan. 8 parliamentary elections into chaos and created fears of mass protests and violence across the nuclear-armed nation, an important U.S. ally in the war on terrorism.
Musharraf condemned the attack and urged calm, according to the state-run Associated Press of Pakistan. He also convened an emergency meeting with his senior staff, where they were expected to discuss whether to postpone the elections, an official at the Interior Ministry said, speaking on condition of anonymity because of the sensitivity of the talks.
No one claimed responsibility for the killing but suspicion was almost certain to fall on Islamic militants.
The attacker struck just minutes after Bhutto addressed thousands of supporters in the garrison city of Rawalpindi, 8 miles south of Islamabad. She was shot in the neck and chest by the attacker, who then blew himself up, said Rehman Malik, Bhutto's security adviser.
Sardar Qamar Hayyat, a leader from Bhutto's party, said he was standing about 10 yard away from Bhutto's vehicle at the time of the attack.
"She was inside the vehicle and was coming out from the gate after addressing the rally when some of the youths started chanting slogans in her favor. Then I saw a smiling Bhutto emerging from the vehicle's roof and responding to their slogans," he said.
"Then I saw a thin, young man jumping toward her vehicle from the back and opening fire. Moments later, I saw her speeding vehicle going away," he added.
Bhutto was rushed to the hospital and taken into emergency surgery. She died about an hour after the attack.
"At 6:16 p.m., she expired," said Wasif Ali Khan, a member of Bhutto's party who was at Rawalpindi General Hospital.
"The surgeons confirmed that she has been martyred," Bhutto's lawyer Babar Awan said.
Bhutto's supporters at the hospital exploded in anger, smashing the glass door at the main entrance of the emergency unit. Others burst into tears. One man with a flag of Bhutto's Pakistan People's Party tied around his head was beating his chest.
"I saw her with my own eyes sitting in a vehicle after addressing the rally. Then, I heard an explosion," Tahir Mahmood, 55, said sobbing. "I am in shock. I cannot believe that she is dead."
Many chanted slogans against Musharraf, accusing him of complicity in her killing.
"We repeatedly informed the government to provide her proper security and appropriate equipment ... but they paid no heed to our requests," Malik said.
As news of her death spread, angry supporters took to the streets in the northwestern city of Peshawar as well other areas, chanting slogans against Musharraf. In Rawalpindi, Bhutto's supporters burned election posters from the ruling party and attacked police, who fled the scene.
In Karachi, shop owners quickly closed their businesses as supporters from Bhutto's party burned tires on the roads.
Nawaz Sharif, another former premier and opposition leader, arrived at the hospital and sat silently next to Bhutto's body.
"Benazir Bhutto was also my sister, and I will be with you to take the revenge for her death," he said. "Don't feel alone. I am with you. We will take the revenge on the rulers."
Speaking to the BBC, Sharif also questioned whether to hold the elections.
"I think perhaps none of us is inclined to think of the elections," he said. "We would have to sit down and take a very serious look at the current situation together with the People's Party and see what we have to do in the coming days."
Hours earlier, four people were killed at a rally for Sharif when his supporters clashed with backers of Musharraf near Rawalpindi.
Bhutto's death will leave a void at the top of her party, the largest political group in the country, as it heads into the elections. It also fueled fears that the crucial vote could descend into violence.
Pakistan is considered a vital U.S. ally in the fight against al-Qaida and other Islamic extremists including the Taliban. Osama bin Laden and his inner circle are believed to be hiding in lawless northwest Pakistan along the border with Afghanistan.
In Washington, the State Department condemned the attack.
"It demonstrates that there are still those in Pakistan who want to subvert reconciliation and efforts to advance democracy," deputy spokesman Tom Casey said.
The United States has for months been encouraging Musharraf to reach an accommodation with the opposition, particularly Bhutto, who was seen as having a wide base of support in Pakistan. Her party had been widely expected to do well in next month's elections.
Pakistan was just emerging from another crisis after Musharraf declared a state of emergency on Nov. 3, and used sweeping powers to round up thousands of his opponents and fire Supreme Court justices. He ended emergency rule Dec. 15 and subsequently relinquished his role as army chief, a key opposition demand. Bhutto had been an outspoken critic of Musharraf's imposition of emergency rule.
Educated at Harvard and Oxford universities, Bhutto served twice as Pakistan's prime minister between 1988 and 1996. Her father, who also served as prime minister, was executed in 1979 two years after his ouster in a military coup.
Bhutto had returned to Pakistan from an eight-year exile on Oct. 18. On the same day, she narrowly escaped injury when her homecoming parade in Karachi was targeted in a suicide attack that killed more than 140 people.
Islamic militants linked to al-Qaida and the Taliban hated Bhutto for her close ties to the Americans and support for the war on terrorism. A local Taliban leader reportedly threatened to greet Bhutto's return to the country with suicide bombings.
At the scene of today's bombing, an Associated Press reporter saw body parts and flesh scattered at the back gate of the Liaqat Bagh park, where Bhutto had spoken. He counted about 20 bodies, including police, and could see many other wounded people.
Police cordoned off the street with white and red tape, and rescuers rushed to put victims in ambulances as people wailed nearby.
The clothing of some victims was shredded and people put party flags over their bodies. Police caps and shoes littered the asphalt.
Hundreds of riot police had manned security checkpoints around the venue. It was Bhutto's first public meeting in Rawalpindi since she came back to the country.
In November, Bhutto had also planned a rally in the city, but Musharraf forced her to cancel it, citing security fears.
In recent weeks, suicide bombers have repeatedly targeted security forces in Rawalpindi, where Musharraf stays and the Pakistan army has its headquarters.