Benazir Bhutto's supporters say the Pakistani government's account of how she died is "dangerous nonsense".
Benazir Bhutto's supporters are unsure who to blame for her murder
A government spokesman said her head was slammed against her vehicle by the blast from a bomb - but colleagues said she died from bullet wounds.
A Pakistani militant, accused by the government of being behind the killing, has denied any involvement.
President Musharraf has ordered firm action to prevent more of the rioting that followed Ms Bhutto's death.
Mr Musharraf said looters "must be dealt with firmly and all measures be taken to ensure (the) safety and security of the people", the official APP news agency reports.
He was speaking to the country's senior security officials including the head of the army.
Troops have been out on the streets on Saturday to try to quell violence.
At least 31 people have died in widespread civil unrest since her assassination in Rawalpindi on Thursday.
The shells of burned cars littered the deserted streets of Ms Bhutto's home city of Larkana on Saturday, after overnight rioting.
Ms Bhutto was buried on Friday at her family's marble mausoleum.
Her widower, Asif Ali Zardari, said the family had been "devastated" by her assassination as "somehow we were hoping we would succeed and they would not".
He said he would reveal the contents of her will on Sunday, which would help clarify the future of her Pakistan People's Party.
Asked whether he wanted to lead the PPP, Mr Zardari told the BBC that would be "the party's decision".
Pakistan's main opposition leader, Nawaz Sharif, visited the family of his former rival on Saturday to pay his respects.
The assassinated leader's supporters chanted "Long live Bhutto!" and anti-government slogans as he arrived.
Citing what it said was an intercepted phone call, the interior ministry said Benazir Bhutto's killing had been ordered by an "al-Qaeda leader", Baitullah Mehsud.
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.
But a spokesman for the South Waziristan tribal leader denied he was involved, calling it "government propaganda".
"He had no involvement in this attack," spokesman Maulana Omar said in a telephone call.
"It is against tribal tradition and custom to attack a woman," he added.
Al-Qaeda and pro-Taleban militants are believed to have been behind dozens of bomb attacks in Pakistan in recent years, in which hundreds of people have died - including many women.
'Soaked in blood'
The government presented video of Benazir Bhutto's last moments at a news conference.
The interior ministry said the primary cause of Ms Bhutto's death appeared to have been a knock on her head as she tried to duck through a sun roof back into her vehicle, and not bullets or shrapnel.
A surgeon who treated her, Dr Mussadiq Khan, said earlier she may have died from a shrapnel wound.
But Ms Bhutto's associates disputed the official account, saying the government was trying to abdicate its responsibility for her security.
"To hear that Ms Bhutto fell from an impact from a bump on a sun roof is absolutely rubbish. It is dangerous nonsense, because it implies there was no assassination attempt," a spokeswoman for Ms Bhutto's PPP party, Sherry Rehman, told the BBC.
"There was a clear bullet wound at the back of the neck. It went in one direction and came out another... My entire car is coated with her blood, my clothes, everybody - so she did not concuss her head against the sun roof."
The interior ministry spokesman, Brig Cheema, insisted that all possible security arrangements had been put in place for Ms Bhutto, and that if she had stayed inside the vehicle she might have survived.
The BBC's security correspondent, Frank Gardner, says it is too early to establish the truth of how Benazir Bhutto died.
There have now been so many conflicting versions of 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.
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.
Ms Bhutto's PPP party says it has still not decided whether to contest elections due on 8 January.
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
Opposition leader Nawaz Sharif said his party would boycott them.
The BBC's Owen Bennett-Jones, at the Bhutto family home in Naudero, southern Pakistan, says that given the level of violence it seems surprising that the government is still planning elections for 10 days' time.
But the authorities are saying they are consulting the political parties and it may well be that they are hoping to secure a consensus about a postponement before making any public announcement, our correspondent adds.
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.
In Washington, a US state department spokesman said the elections should go ahead on schedule if they could be held smoothly and safely.
A spokesman for President Musharraf has said it is too early to decide.