One man was killed in a shootout between police and protesters in Tando Allahyar, a town 120 miles north of Karachi, said Mayor Kanwar Naveed. In the town of Tando Jam, protesters forced passengers to get out of a train and then set it on fire.

Two people were killed in the southern Sindh province and two others in Lahore, police said.

Violence also broke out in Lahore, Multan, Peshawar and many other parts of Pakistan, where Bhutto's supporters burned banks, state-run grocery stores and private shops. Some set fire to election offices for the ruling party, according to Pakistani media.

Akhtar Zamin, home minister for the southern Sindh province, said authorities would deploy troops to stop violence if needed.

Musharraf urged calm.

"I want to appeal to the nation to remain peaceful and exercise restraint," he said.

Sharif 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."

He rebutted suggestions that he could gain political capital from her demise, announcing his Muslim League-N party would boycott the elections and demanding that Musharraf resign.

"The holding of fair and free elections is not possible in the presence of Pervez Musharraf," he said. "Musharraf is the cause of all the problems. The federation of Pakistan cannot remain intact in the presence of President Musharraf," he told a news conference.

"After the killing of Benazir Bhutto, I announce that the Pakistan Muslim League-N will boycott the elections," Sharif said. "I demand that Musharraf should quit immediately."

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.

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.

The U.S. has invested significant diplomatic capital in promoting reconciliation between Musharraf and 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.

Had the PPP either won a majority of seats or enough to put together a majority coalition, Bhutto could have recaptured the job of prime minister.

Bush, speaking briefly to reporters at his ranch in Crawford, Texas, demanded that those responsible for the killing be brought to justice.