Ms Bhutto was rushed from the scene to her Karachi residence
Ms Bhutto, who was travelling from the city's airport to a rally marking her homecoming after eight years in self-imposed exile, was not hurt.
The truck carrying her had its windows shattered and a door blown off.
The attacks on the motorcade happened despite a heavy security presence following threats from militant groups.
Witnesses described horrific scenes, with bodies and body parts littering the area. Children were among the dead.
Several Islamist groups, including pro-Taleban militants, had said they would attack Ms Bhutto on her return, after she promised to confront those operating in the northern tribal areas bordering Afghanistan.
Her husband, Asif Ali Zardari, said members of the government and intelligence agencies who were going to lose power were behind the attack.
Ms Bhutto had earlier warned that if targeted, she would hold what she described as hidden authorities within the government as partly responsible.
Extremists will not be allowed to stop Pakistanis from selecting their representatives through an open and democratic process
Gordon Johndroe
White House Spokesman
In quotes: Blast reaction From joy to horror Send us your comments
Ms Bhutto heads the country's largest political force, the Pakistan People's Party (PPP).
She wants to contest parliamentary elections due to be held in January and she has been negotiating with President Pervez Musharraf over a possible power-sharing agreement.
The US has backed such a deal, amid concerns about the military's inability to defeat Islamist militants and Gen Musharraf's rising unpopularity.
Ms Bhutto has been prime minister twice. On both occasions, her government was prematurely dismissed by the president of the day under special powers.
She left Pakistan in April 1999, shortly before Gen Musharraf seized power in a coup - and two years after her husband was jailed and a series of corruption charges were brought against her. She denies the charges.
Gen Musharraf said the attack on Ms Bhutto's convoy was a "conspiracy against democracy".
"The president appealed to the nation and especially the people of Karachi to exercise patience and calm in this hour," said a statement by his office.
The United States also condemned the blasts.
"Extremists will not be allowed to stop Pakistanis from selecting their representatives through an open and democratic process," said National Security Council spokesman Gordon Johndroe.
'Suicide bomber'
Victoria Schofield, a friend of Ms Bhutto who was also on the bus, said the former prime minister had been standing on top of the bus for six hours, but had just gone downstairs to work on a speech when the first explosion went off.
"We were sitting up on the top and suddenly there was this absolute flash of light and a blast," she told the BBC.
There was blood all around and it was chaos - we didn't know where to go, what to do
Farzana Raja
PPP spokeswoman
On board Bhutto's flight In pictures: Blast scene
"I felt lots of really hot air coming and we all - there were about 15 of us on the top of the bus - we all literally went to the ground."
Ms Schofield said the first blast was relatively small but that it had had left people sitting on the left-hand side of the bus covered with blood.
She said the first explosion had come from a parked car which police had begun to investigate. Ms Schofield said a second, much larger explosion occurred two minutes later.
"There was blood all around and it was chaos - and we couldn't understand what was happening - we didn't know where to go, what to do," said Farzana Raja, a spokeswoman for the PPP.
Most of the dead were members of the PPP, although police vehicles took the main force of the blasts and more than 20 police officers are thought to have died. A cameraman for a local TV station was also killed.
KEY DATES 06 Oct: Presidential polls held 17 Oct: Supreme Court resumes hearing challenges to Musharraf candidacy 18 Oct: Benazir Bhutto's homecoming 15 Nov: Parliamentary term ends and general election must be held by mid-January
Flying into uncertainty
The chief of police in Karachi, Azhar Farooqi, said a preliminary investigation suggested the second blast had been caused by a suicide bomber.
"The first blast was probably a hand grenade and it did not cause much damage," he told the BBC.
After the blasts, a dazed Ms Bhutto was immediately rushed from the scene to her Karachi home.
Ms Bhutto flew in from Dubai earlier on Thursday, accompanied by about 100 PPP members.
At least 200,000 people turned out to greet her in what correspondents described as a carnival atmosphere, but the crowds slowed the progress of her convoy.
Ms Bhutto had been planning to make a speech at the tomb of Pakistan's founding father, Mohammed Ali Jinnah.
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Vermutlich Selbstmordattentäter hatten bei den Feierlichkeiten zur Rückkehr der ehemaligen pakistanischen Premierministerin Benazir Bhutto zwei Explosionen ausgelöst. Der Anschlag forderte 136 Tode, mindestens 387 Personen erlitten Verletzungen. 

Unter den Opfern waren viele Angehörige von Bhuttos Volkspartei, ein Kameramann eines lokalen Fernsehsenders und zwanzig Sicherheitsoffiziere. Präsident Pervez Musharraf nannte den Anschlag eine „Verschwörung gegen die Demokratie“.

Nach einer ersten kleinen Explosion kam es zu einer weiteren Zündung vor dem Vehikel Bhuttos. Dabei wurden die Scheiben und Türen zerstört. Bhutto selbst blieb unverletzt und wurde eilig in Sicherheit gebracht. Polizeiwagen nahmen den Großteil der Explosion auf.

Ein Sprecher der PPP (Bhuttos Pakistanische Volkspartei, englisch: „Pakistan People's Party“) berichtete, dass überall Blut und Chaos war. Man habe nicht gewusst, was los war und was man tun sollte.

Bhutto schloss aus, dass das Attentat ihr als Person gegolten habe. Der Angriff habe vielmehr der Demokratie sowie der Einheit und Integrität Pakistans gegolten. Als die für sie gefährlichsten Gruppen benannte die Politikerin die Al Qaida, die Taliban und eine Gruppe von Selbstmordattentätern, die Bhutto jedoch nicht näher benannte. Sie verlangte eine Untersuchung, warum die Lichter ausgeschaltet waren, wodurch es schwieriger war, Personen zu kontrollieren. Einzelne Leute würden Machtmissbrauch betreiben, vermutet die 54-Jährige.