Citing what it said was an intercepted phone call, the interior ministry said the killing had been ordered by an "al-Qaeda leader", Baitullah Mehsud.

The BBC's security correspondent, Frank Gardner, says it is too early to establish the truth of what happened.

Ms Bhutto has been buried in her family tomb amid scenes of mass grieving.

Video of her last moments before the attack in Rawalpindi was shown at the news conference given in Islamabad by the interior ministry.

According to the ministry, the primary cause of Ms Bhutto's death appears to have been a knock on her head as she tried to duck her attacker, and not bullets or shrapnel. Her party denies this.

Pakistani security forces are on high alert, with at least 31 people killed in protests by Bhutto supporters across the country since the assassination.

Conflicting versions

Baitullah Mehsud is a tribal leader in Pakistan's South Waziristan region.

Pakistani intelligence services intercepted a call from him in which he allegedly congratulated another militant after Ms Bhutto's death, interior ministry spokesman Javed Iqbal Cheema told reporters.

There was, he added, "irrefutable evidence that al-Qaeda, its networks and cohorts were trying to destabilise Pakistan".

There have now been so many conflicting versions coming out of Pakistan of how Benazir Bhutto died and who sent the assassin that it is hard for anyone to build up an accurate picture, our security correspondent says.

Both al-Qaeda and the Taleban are perfectly plausible culprits since they hated everything the secular Ms Bhutto stood for, he adds.

But critics of President Pervez Musharraf are unlikely to be convinced by his government's insistence that it has proof al-Qaeda ordered the murder.

'Pack of lies'

Brig Cheema said Ms Bhutto had smashed her head against a lever of her car's sun roof.

She was, he said, trying to shelter inside the car from the gunman, who set off a bomb after opening fire with a gun.

A surgeon who treated her, Dr Mussadiq Khan, said earlier she may have died from a shrapnel wound while Ms Bhutto's security adviser, Rehman Malik, said she had been shot in the neck and chest.

Farooq Naik, a senior official in Ms Bhutto's Pakistan People's Party, said the government's explanation of her death was a "pack of lies".

"Two bullets hit her, one in the abdomen and one in the head," he told AFP news agency.

Brig Cheema added that all possible security arrangements had been put in place for Ms Bhutto.

Her supporters say the government did not do enough to protect her.

After a previous attempt on her life in October that killed 130 people, Ms Bhutto accused rogue elements of the Pakistani intelligence services of involvement.

Unrest

Ms Bhutto was buried next to her father Zulfikar Ali Bhutto in the family mausoleum near their home village, Garhi Khuda Bakhsh, in Sindh province, as thousands of mourners attended.

Rioters in Peshawar shouted slogans against President Musharraf

Many European and Asian countries have warned their citizens against travelling to Pakistan because of concern that the killing of Benazir Bhutto could provoke more violence.

Rioting and unrest have been reported across the country.

Six bodies were found among the remains of a factory set on fire in Karachi

At least one passenger train was set ablaze in Sindh Province and a number of railway stations were reportedly burnt as security forces in the province were ordered to shoot rioters on sight

In the city of Multan in Punjab province, a mob ransacked seven banks and torched a petrol station

Other cities across Pakistan are at a virtual standstill.

Schools, businesses and transport are all closed, and people are reluctant to step out during the three days of national mourning declared by Mr Musharraf.

In another development, a bomb attack on an election meeting of the pro-Musharraf Pakistan Muslim League in Swat, north-western Pakistan, killed at least nine people including a candidate on Friday.

Election questions

A spokesman for the President Musharraf has said it is too early to decide whether the parliamentary election on 8 January should be postponed.

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

Mohammad Mian Soomro, the caretaker prime minister, urged all political parties to talk to the government, so that a decision could be reached by consensus.

The election is meant to pave the way for a return to democratic rule, suspended in October 1999 when the then Gen Musharraf seized power through a coup.

But opposition parties are now against the election taking the place and it is hard to see how they it would be a true test of the democratic process, the BBC's Karishma Vaswani reports.

Ms Bhutto returned from eight years of self-imposed exile in October, following an amnesty agreed with President Musharraf.

Speaking in Washington, US Secretary of State Condoleezza Rice called for the democratic process in Pakistan to continue, without commenting on the January election date.

"The way to honour [Benazir Bhutto's] memory is to continue the democratic process in Pakistan so that the democracy that she so hoped for can emerge," she said.