KARACHI, Pakistan (CNN) -- At least 136 people were killed and more than 387 wounded around midnight Thursday in a suicide bombing near a motorcade carrying former Pakistani Prime Minister Benazir Bhutto, who returned to the country earlier in the day after eight years of self-imposed exile, according to hospital and police sources.

Bhutto, recently returned to Pakistan, is helped from her vehicle following the bombing. more photos »

Bhutto and those with her were unhurt, and her companions said she reached her family home safely. Video footage showed her exiting the bullet- and blast-proof vehicle after the blasts.

Pakistan's president, Gen. Pervez Musharraf, called Bhutto Friday to condemn the bombing and assure her that an independent investigation will be completed as soon as possible, his office said.

She apparently had moved from the roof of the vehicle inside and downstairs just moments before the blasts.

"I can see body parts strewn all over the road," said CNN's Dan Rivers, at the scene. "There are dead bodies everywhere. ... It is a large-scale attack, by the looks of things."

Authorities believe the suicide bomber was on foot and threw a grenade to attract attention before setting of the second, major blast, Karachi police chief Azhar Farooqi told CNN. The bomber is believed to have acted alone.

Police do not think a car bomb was involved, he said. Nearby cars were burned but police do not believe a bomb was inside the car.

He would not say who authorities believe was behind the bombing, citing the ongoing investigation.

"Although the truck that Benazir Bhutto was riding on was surrounded by police cars, so the suicide bomber could not get onto the truck and could not get anywhere near it, so he blew himself up and that has caused many casualties, mostly among the policemen who were riding beside the truck," Tariq Azim Khan, Pakistani information minister, told CNN.

Other officials said at least one bomb apparently had been placed in a car on the street where Bhutto's supporters had gathered to see her convoy pass. One witness told Rivers he saw a car with three people inside explode. Watch witnesses describe what happened »

Video footage showed a chaotic scene after the explosions, with crowds of people trying to flee as emergency vehicles jammed streets. Other footage showed wounded victims writhing on a road, awaiting medical attention, and at least one fire apparently sparked by the blasts.

The windshield of the vehicle Bhutto was riding in was smashed by the blasts, Rivers said, and a vehicle that was following hers was burned out. The scene, he said, was "absolutely horrendous," with blood running in streams down the street.

Because the streets were crowded with supporters who had turned out to greet Bhutto, ambulances had difficulty reaching the scene immediately after the blasts. Onlookers resorted to ferrying the injured to hospitals in private cars.

Rivers said he and his crew, filming the convoy just before the blasts, remarked on the lack of security surrounding it. It was possible to walk right up to the side of her vehicle without being stopped, he said.

Qasim Zia, a leader of Bhutto's Pakistan People's Party, was riding on Bhutto's vehicle and told CNN one of his bodyguards was killed and another seriously hurt. The wounded included at least 20 leaders of the party, he said, and most of those killed were members of security forces or police who were surrounding Bhutto's truck at the time of the explosions.

Had it not been for heightened security measures in place, Bhutto could have been wounded or killed, he said.

The bomb detonated as Bhutto's motorcade was nearing the tomb of Muhammad Ali Jinnah, who led Pakistan to independence and championed equal rights for all Pakistani citizens regardless of their religion. Bhutto had planned to stop and pray at the tomb, then deliver a speech to her supporters.

The United States was swift to condemn what it called "terrorist attacks in Karachi during peaceful political demonstrations."

"There is no political cause that can justify the murder of innocent people," State Department spokesman Tom Casey said in a written statement.

White House spokesman Gordon Johndroe said, "extremists will not be allowed to stop Pakistanis from selecting their representatives through an open and democratic process."

United Nations Secretary-General Ban Ki-moon also issued a statement condeming the bombing, and British Foreign Secretary David Miliband said he was "appalled" by the "horrific" attacks.

The terrorist watch group IntelCenter said the death toll from the bombing places it among the top 10 deadliest terror attacks within the past nine years. E-mail to a friend

CNN's Syed Mohsin Naqvi contributed to this report.

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