In Peshawar, protesters also burned the office of a small party allied with Musharraf.
Other areas were nearly deserted Friday morning as businesses closed and public transportation came to a halt at the start of three days of national mourning for the opposition leader.
"The repercussions of her murder will continue to unfold for months, even years," read a mournful editorial in the Dawn newspaper. "What is clear is that Pakistan's political landscape will never be the same, having lost one of its finest daughters."
As many Pakistanis mourned, others demanded to learn who killed her. Musharraf blamed the attack on the resurgent Islamic militants Pakistan is fighting along the border region with Afghanistan, pledging in a nationally televised speech that "we will not rest until we eliminate these terrorists and root them out."
But authorities said they had yet to identify the attacker. "It is too early to say who may have been responsible," said Saud Aziz, the chief of police in Rawalpindi, the city near Islamabad where the attack took place. A joint task force of police and officials from other law enforcement agencies were investigating, he said.
FBI spokesman Richard Kolko in Washington said the agency was trying to determine the validity of a purported claim of responsibility for the attack by al-Qaida.
President Bush, who spoke briefly by phone with Musharraf, looked tense as he spoke to reporters, denouncing the "murderous extremists who are trying to undermine Pakistan's democracy."
In the wake of the killing, Nawaz Sharif, another former premier and leader of a rival opposition party, announced his party would boycott the elections.
The election was seen as a pivotal step toward restoring democracy here, eight years after Musharraf seized power in a coup. It also was intended to restore credibility to the government after Musharraf used a six-week state of emergency to arrest thousands of political opponents and crack down on the independent judiciary.
However, with Sharif's party on the sidelines and Bhutto's party leaderless and in disarray, the election will have little, if any, credibility. "This assassination is the most serious setback for democracy in Pakistan," said Rasul Baksh Rais, a political scientist at Lahore's University of Management Sciences. "It shows extremists are powerful enough to disrupt the democratic process."
Sharif demanded Musharraf's resignation. "Musharraf is the cause of all the problems," Sharif said. Bhutto's death closed another grim chapter in Pakistan's bloodstained history, 28 years after her father, Zulfiqar Ali Bhutto, another ex-prime minister, was hanged by a military dictatorship just a few miles from where she was killed.
As the news of her killing spread, supporters gathered at the hospital where Bhutto had been taken, smashed glass doors, stoned cars and chanted, "Killer, Killer, Musharraf."
The U.N. Security Council vigorously denounced the killing and urged "all Pakistanis to exercise restraint and maintain stability in the country."
The United States, meanwhile, struggled to reformulate its plan to stabilize the country based on a rapprochement between Bhutto and Musharraf.
Next to Musharraf, Bhutto, 54, was the country's best known political figure, serving two terms as prime minister between 1988 and 1996. She was respected in the West for her liberal outlook and determination to combat Islamic extremism.