Benazir Bhutto vowed today to push on with her political comeback despite a devastating suicide bomb attack that killed more than 130 people at her homecoming procession last night and injured several hundred others.

The former premier gave details of the attack at a press conference in Karachi today and said that she had already given the names of three people she believed to be responsible to President Pervez Musharraf.

Ms Bhutto said that two men had carried out the attack and her security guards had found a third man armed with a pistol and a fourth with a suicide vest.

Although she was not blaming Mr Musharraf's government "at this stage", Ms Bhutto demanded an inquiry into why the street lights had been switched off on the road along which her motorcade was travelling, obscuring the attackers as they approached her truck through the huge crowds.

Hundreds of thousands of people had packed the streets of the city to welcome her home after years of self-imposed exile.

Wearing a white headscarf and traditional tunic and trousers as well as a black armband, the 54-year-old opposition leader said, however, that the attacks would not deter her from leading the Pakistan People's Party in parliamentary elections due in January.

"We are prepared to risk our lives. We’re prepared to risk our liberty. But we’re not prepared to surrender this great nation to militants," she said. "The attack was on what I represent. The attack was on democracy and very unity and integrity of Pakistan."

The blast was thought to be Pakistan's most deadly bombing and left Karachi, a port city of 14 million people, in stunned silence.

Schools and colleges were ordered to remain closed and the city's normally packed six-lane highways were all but deserted. Ms Bhutto’s plans to make her first big public speech early today at the mausoleum of Muhammad Ali Jinnah, founder of Pakistan, were scrapped.

Police said today that they had found the severed head of the bomber, a young man who had been carrying some 20kg of explosives, and were trying to identify him. The Interior Ministry said that the first blast had been caused by a hand grenade - although there were conflicting accounts.

In an interview with Paris Match magazine only a few hours after the bombing, Ms Bhutto said that her entourage had been tipped off about a potential suicide attack about half an hour before it happened. She said that the police had sent in reinforcements to help protect her convoy - many of whom died in the attack.

Nevertheless, she accused high-ranking members of Pakistan's intelligence services of being behind the attack, arguing that Islamic militants could not mount such a sophisticated attack "from a mountain cave".

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