RAWALPINDI, Pakistan — Pakistan opposition leader Benazir Bhutto was assassinated Thursday by an attacker who shot her after a campaign rally and then blew himself up. Her death stoked new chaos across the nuclear-armed nation, an important U.S. ally in the war on terrorism.

At least 20 others were killed in the attack on the rally for Jan. 8 parliamentary elections where the 54-year-old former prime minister had just spoken.

At least five people were killed across the country in rioting that broke out in the aftermath of the assassination. In the southern port city of Karachi, angry Bhutto supporters shot at police and burned a gas station.

At the hospital where Bhutto died, some supporters smashed glass and wailed, chanting slogans against President Pervez Musharraf. Musharraf blamed Islamic extremists for her death and said he would redouble his efforts to fight them.

"This is the work of those terrorists with whom we are engaged in war," he said in a nationally televised speech. "I have been saying that the nation faces the greatest threats from these terrorists. ... We will not rest until we eliminate these terrorists and root them out."

In the U.S., a tense looking President Bush strongly condemned the attack "by murderous extremists who are trying to undermine Pakistan's democracy." White House spokesman Scott Stanzel said Bush spoke briefly by phone with Musharraf.

Musharraf 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.

The government announced three days of mourning for Bhutto, including the closing of schools, commercial centers and banks.

Nawaz Sharif, another former prime minister and leader of a rival opposition party, demanded Musharraf resign immediately and announced his party would boycott the upcoming election.

The attacker struck just minutes after Bhutto addressed thousands of supporters in the Rawalpindi, a city 8 miles south of Islamabad where the army is headquartered. 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 at the time of the attack he was standing about 10 yards away from her vehicle — a white, bulletproof SUV with a sunroof.