Benazir Bhutto had been addressing rallies in many parts of Pakistan
Ms Bhutto - the first woman PM in an Islamic state - 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.
Benazir Bhutto's coffin has now been taken from the hospital
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.
Scene of grief
Ms Bhutto's coffin was removed from hospital in Rawalpindi and has now arrived by plane in Sukkur in Sindh province for burial in her home town, Larkana.
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Her husband, Asif Ali Zardari, has arrived in Pakistan from Dubai to escort the coffin to its final resting-place.
The attack occurred close to an entrance gate of the city park where Ms Bhutto had been speaking.
Police confirmed reports Ms Bhutto had been shot in the neck and chest before the gunman blew himself up.
She died at 1816 (1316 GMT), said Wasif Ali Khan, a member of the PPP who was at hospital.
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Some supporters at the hospital wept while others broke into anger, throwing stones at cars and breaking windows.
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.
In other violence:
Police in Peshawar, in the north-west, used batons and tear gas to break up a rally by protesters chanting anti-Musharraf slogans
One man was killed in a "shoot-out" between police and protesters in Tando Allahyar, the mayor said
Unrest was also reported in Quetta, Multan and Shikarpur
'Security lapse'
Mr Musharraf has announced three days of national mourning. All schools, colleges, universities, banks and government offices will remain closed.
Protesters set vehicles on fire in the streets of Hyderabad
Mr Sharif said there had been a "serious lapse in security" by the government.
Earlier on Thursday, at least four people were killed ahead of an election rally Mr Sharif had been preparing to attend close to Rawalpindi.
Ms Bhutto's death has plunged the PPP into confusion and raises questions about whether January elections will go ahead as planned, the BBC's Barbara Plett in Islamabad says.
The killing was condemned by India, the US, the UK and others.
US President George W Bush telephoned Mr Musharraf for what the White House would only describe as a "brief" conversation on the situation.
BENAZIR BHUTTO Father led Pakistan before being executed in 1979 Spent five years in prison Served as PM from 1988-1990 and 1993-1996 Sacked twice by president on corruption charges Formed alliance with rival ex-PM Nawaz Sharif in 2006 Ended self-imposed exile by returning to Pakistan in October Educated at Harvard and Oxford
Obituary: Benazir Bhutto Life in pictures
Her return was the result of a power-sharing agreement with President Musharraf
He had granted an amnesty that covered the court cases she was facing.
But relations with Mr Musharraf soon broke down.
On the day of her arrival, she had led a motor cavalcade through the city of Karachi.
It was hit by a double suicide attack that left some 130 dead.
Rawalpindi, the nerve centre of Pakistan's military, is seen as one of the country's most secure cities.
Many analysts say attacks like those on Thursday show the creeping "Talebanisation" of Pakistan.
Radical Muslims calling for Islamic law, and fiercely opposed to the US, have become increasingly active in Pakistani politics in recent years, analysts say.
Benazir Bhutto(21 de junio, 1953 – 27 de diciembre, 2007)


Benazir Bhutto, de 54 años, una ex Primera Ministra de Pakistán y la primera mujer en llegar a dicha posición en un país islámico, murió en un presunto ataque suicida, según indicó Farhatullah Babar, un portavoz del PPP (Partido Popular Pakistaní). Reportes anteriores habían indicando que ella solamente había sido herida y llevada al hospital.

Bhutto fue herida poco después de hablar en una reunión política del PPP en Rawalpindi; instantes luego de su discurso, recibió un disparo en el cuello. Poco después, la persona que disparó detonó una bomba en el lugar. Finalmente, metralla proveniente de la explosión la mató. Al menos 28 personas resultaron muertas y 100 heridas en el incidente.

Farhatullah Babar había declarado que Bhutto solamente había resultado herida. Sin embargo, luego reportó que la líder política había muerto. Adicionalmente, otro portavoz del PPP manifestó a la ''Associated Press'' que "A las 6.16 p.m. (1316 GMT), ella expiró".

Benazir Bhutto volvió a Pakistán en octubre tras un exilio auto-impuesto. La líder política volvió al país tras acuerdos con el actual presidente, Pervez Musharraf.

Pero las relaciones entre ambos políticos se fueron degradando, sobre todo cuando Musharraf declaró el estado de excepción en noviembre. Desde entonces, Bhutto hacía campaña política contra él y a favor del PPP para las elecciones de enero de 2008.

En un reporte de la sede de la ''Associated Press'' en Pakistán, se dijo que el gobierno pakistaní cree que Al Qaeda fue responsable por el ataque, ya que se cree que el atacante era parte de dicha organización, y que ellos reivindicaron el ataque. No se ha podido confirmar dicha reivindicación, ya que no se ha encontrado ningún mensaje de esta naturaleza en los sitios web islamitas extremistas.

Varios líderes internacionales condenaron el ataque como un acto terrorista y expresaron su sorpresa y tristeza con el incidente.

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