(Adds details of Bhutto's killing, updates death toll)
By Faisal Aziz
GARHI KHUDA BAKHSH, Pakistan, Dec 28 Benazir Bhutto was laid to rest next to her father in the family mausoleum on Friday after the opposition leader's assassination plunged Pakistan into crisis and triggered violent protests.
Tens of thousands of mourners wept and beat their heads as Bhutto, killed by a suicide attacker at an election rally on Thursday, was carried from her ancestral home in Sindh province, in the south of the country, to the domed mausoleum.
The 54-year-old's death stoked fears a Jan. 8 election meant to return Pakistan to civilian rule could be put off amid a backlash threatening to engulf embattled President Pervez Musharraf.
Pakistan said there were signs al Qaeda was responsible.
"We have intelligence intercepts indicating that al Qaeda leader Baitullah Mehsud is behind her assassination," an Interior Ministry spokesman said. Mehsud, based near the Afghan border, is one of Pakistan's most wanted militant leaders.
Troops were called out to quell protests in Bhutto's home province of Sindh, where she had huge support, particularly among the rural poor. Officials said 31 people, including four policemen, had been killed since Bhutto's assassination.
Bhutto's husband, Asif Ali Zardari, wept as he accompanied her coffin, draped with the green, red and black tricolour of Bhutto's Pakistan People's Party, on the 7-km (4-mile) journey to the tomb in the dusty village of Garhi Khuda Bakhsh.
He then prayed there with the couple's three children, son Bilawal, 19, and daughters Bakhtawar, 17 and Aseefa, 14.
Many mourners chanted slogans against Musharraf and the United States, which has long backed the former army general in the hope he can maintain stability in the nuclear-armed country racked by Islamist violent.
"Shame on the killer Musharraf, shame on the killer U.S.," mourners cried.
Others wept in despair. "Bhutto was my sister and Bhutto was like my mother," cried farmer Imam Baksh. "With her death, the world has ended for us."
Musharraf, who seized power in a military coup in 1999 but left the army last month to become a civilian president, has appealed for calm and blamed Islamist militants for the killing.
But many accused him of failing to protect Bhutto, who died in the garrison city of Rawalpindi, home of the Pakistani army.
World leaders urged Pakistan to stay the course towards democracy, as Bhutto's death rattled markets and triggered a flight to less risky assets such as bonds and gold.
"Unrest in Pakistan is eroding the market sentiment dramatically as Pakistan, unlike North Korea or Iran, is known to really have nuclear weapons," said Koichi Ogawa, chief portfolio manager at Daiwa SB Investments.
In Sindh, authorities issued an order to shoot violent protesters on sight. Hundreds of cars, trucks and buses smouldered in the interior of the province and crowds of men set up road blocks and chanted slogans against Musharraf.
Meanwhile, a blast at an election meeting in Pakistan's troubled northwest killed six people including a candidate for the party that supports Musharraf, police said.
There were also sporadic protests elsewhere in the country and one person was killed in the eastern city of Lahore.
Bhutto returned home from self-imposed exile in October, hoping to become prime minister for a third time.
But as she left the rally she stood to wave to supporters from the sun-roof of her bullet-proof car. An attacker shot at her before blowing himself up, police and witnesses said.
The Interior Ministry said Bhutto had not been shot, nor hit by shrapnel, but had been killed when the force of the explosion smashed her head against a lever on the sun-roof. Security officials had earlier said she was shot in the head and neck.
She was buried alongside her father, former Prime Minister Zulfikar Ali Bhutto, who was hanged in 1979 after being deposed by a military coup. Her two brothers, Murtaza and Shahnawaz, who both died in unexplained circumstances, are also buried in the mausoleum she herself had ordered to be built.
MUSHARRAF UNDER ATTACK
The United States, which relies on Pakistan as an ally against al Qaeda and the Taliban in neighbouring Afghanistan, had championed the Harvard- and Oxford-educated Bhutto.
Her death dashed U.S. hopes of a power-sharing agreement between her and Musharraf.
President George W. Bush urged Pakistanis to honour Bhutto's memory by going ahead with the election.
"Elections stand as they were announced," Prime Minister Mohammadmian Soomro told reporters. But analysts said the assassination, following a wave of suicide attacks and the worsening of an Islamist insurgency, could make this impossible.
"If it's left to Pervez Musharraf then he will try to ram it through but on the ground it's going to be very difficult," said Talat Masood, a retired general and political analyst.
"Now voices are being raised that he is the problem and not the solution as the Americans think," he said. "He may be a casualty as a result of that."
Former Prime Minister Nawaz Sharif, who was deposed by Musharraf in the 1999 coup, said his party would boycott the January election and blamed Musharraf for the instability.
Musharraf imposed a state of emergency in November in what was seen as an attempt to stop the judiciary from vetoing his re-election as president. He lifted emergency rule this month.
In 1988, aged just 35, Bhutto became the Muslim world's first democratically elected woman prime minister. Deposed in 1990, she was re-elected in 1993, and ousted again in 1996 amid charges of corruption she said were politically motivated.
Bhutto escaped unhurt from a suicide attack in October that killed at least 139 people.
She had spoken of al Qaeda plots to kill her. But she also had enemies in other quarters including among the powerful intelligence services and some allies of Musharraf. (Additional reporting by Kamran Haider, Zeeshan Haider and Robert Birsel; writing by Myra MacDonald; editing by Matthew Tostevin; firstname.lastname@example.org; +92 51 281 0016; Reuters Messaging: email@example.com; edited by Richard Meares))