Ms Bhutto was being driven in a convoy through crowded streets from Karachi airport to a rally to mark her homecoming after eight years in exile.
Ms Bhutto was not among the casualties and has been driven to safety.
Hundreds of thousands of people had turned out to greet the former PM, amid a huge security presence.
Several Islamist groups including pro-Taleban militants have made threats against Ms Bhutto.
The motorcade is now said to be at a standstill, and police have cordoned off the scene of the blasts.
The area around a stage where she was due to give a speech to supporters has been evacuated.
Police say the bombings may have been suicide attacks.
Rushed away
The first explosion was relatively small but was followed by a much larger blast close to the front of Ms Bhutto's truck, which broke its windows and door.
Police vehicles took the main force of the blasts.
It is not clear whether the bomber was in a car or on foot, although police said an unregistered small white car pulled up near a police escort immediately before the explosions.
Witnesses said body parts were strewn across Ms Bhutto's truck. The death toll is expected to rise.
Most of the dead were members of her Pakistan People's Party (PPP). One cameraman for a local TV station was killed.
Correspondents say Ms Bhutto may have been shielded from the blast by an overhang because she was sitting on top of the truck at the time.
The former prime minister was immediately rushed from the scene to her Karachi residency, according to contingency plans.
Power-sharing deal
Ms Bhutto flew in from Dubai earlier on Thursday, accompanied on the flight 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.
The BBC's Damian Grammaticas in Karachi says that despite being away for eight years, Ms Bhutto is still enormously popular as a scion of Pakistan's pre-eminent political dynasty.
Ms Bhutto left the country soon after Gen Musharraf seized power in a coup, but returned under a power-sharing deal with the president which could see her becoming prime minister again.
The US has backed the deal, amid concerns about the military's inability to defeat Islamist militants and Gen Musharraf's rising unpopularity.
