Benazir Bhutto, the eldest child of Zulfiqar Ali Bhutto ,was leaving an election rally in Rawalpindi when a gunman shot her in the neck and set off a bomb. At least 20 other people died in the attack and several more were injured.
President Pervez Musharraf has urged people to remain calm but angry protests have gripped some cities, with at least 11 deaths reported.Security forces have been placed on a state of "red alert" nationwide.
There were no immediate claims of responsibility for the attack. Analysts believe Islamist militants to be the most likely group behind it.
Ms Bhutto, leader of the Pakistan People's Party (PPP), had served as prime minister from 1988-1990 and 1993-1996, and had been campaigning ahead of elections due on 8 January.
It was the second suicide attack against her in recent months and came amid a wave of bombings targeting security and government officials.
Nawaz Sharif, also a former prime minister and a political rival, announced his Muslim League party would boycott the elections. He called on President Musharraf to resign, saying free and fair elections were not possible under his rule.
The United Nations Security Council held an emergency session and later said it "unanimously condemned" the assassination.
Police has confirmed reports Ms Bhutto had been shot in the neck and chest before the gunman blew himself up.She died at 18.16 said Wasif Ali Khan, a member of the PPP who was at hospital.
Protests erupted in other cities as news of the assassination spread, with reports of 11 deaths in the PPP's heartland province of Sindh, including four in provincial capital, Karachi. More than 100 cars were burned in Karachi, while cars and a train were reportedly set on fire in Hyderabad.
1. Benazir Bhutto had addressed a rally of thousands of supporters in Rawalpindi's Liaqat Bagh Park
2. As her convoy was leaving the park via the rear gate onto Murree road, she was shot twice in the neck and chest
3. The gunman then blew himself up killing at least 16 people
4. Ms Bhutto was taken to Rawalpindi General Hospital, but was pronounced dead at 1816 local time.