Al-Qaeda's top commander in Afghanistan has claimed responsibility for the assassination of Pakistan opposition leader Benazir Bhutto, the Asia Times Online reported Friday.

In a telephone conversation with the news site, Mustafa Abu al-Yazid, who emerged in May as the chief of al-Qaeda's operations in Afghanistan, claimed to have ordered the operation to kill Bhutto, who was shot dead Thursday after speaking at a political rally in the Pakistani city of Rawalpindi.

"This is our first major victory against those who have been siding with infidels in a fight against al-Qaeda and declared a war against mujahedin," Mustafa was quoted as saying.

He referred to recent campaign addresses by Bhutto ahead of January 8 parliamentary elections in which she lambasted Islamic extremism.

The militant commander told Asia Times that a death squad consisting of members of the Pakistan-based militant Islamic group Laskhar-i-Jhangvi carried out the assassination on al-Qaeda's orders.

The claim of responsibility came as Pakistan prepared Friday to bury Bhutto, whose assassination sparked nationwide riots and the country's most serious political crisis in decades.

Laskhar-i-Jhangvi, based in Pakistan's Punjab region, is a known collaborator with both the Taliban and al-Qaeda. The group attempted to assassinate Nawaz Sharif in January 1999 when he was prime minister, and is believed to have assisted in several high-profile attacks on Westerners in Pakistan, including the January 2002 kidnapping and murder of Wall Street Journal reporter Daniel Pearl, according to counter-terrorism sources.

There was no independent confirmation of the report and Pakistan Interior Ministry spokesman Javed Iqbal Cheema said it was too early to conclude that al-Qaeda was behind the assassination.

"All we know is that those who carried out this attack are the same people who are perpetrating terrorist attacks across the country," he said.

Pakistan has seen more than 50 terrorist attacks targeting the government and military in 2007, which Musharraf has blamed on the Taliban, al-Qaeda and other extremist groups that have regrouped in the North-West Frontier Province's lawless tribal areas. There has also been a sharp surge in Taliban-led violence in Afghanistan this year, with 140 suicide attacks.

The instability in western Pakistan has directly affected security in Afghanistan, US officials say, and presents a nightmare scenario for President George W Bush's administration. US officials concede that al-Qaeda has regrouped inside Pakistan and is not only launching attacks there and in Afghanistan but also has the freedom to plan future attacks on the United States.

Musharraf, a critical ally in the Bush administration's war on terrorism, has vowed to root out violent Islamic extremists from the tribal belt bordering Afghanistan and during an address to the nation on Thursday night said Bhutto's assassination "was the action of the same terrorists against whom we are at war."

Meanwhile, parts of Pakistan were in danger of spinning out of control Friday as angry mobs continued attacks on government buildings and private property following Bhutto's assassination.

The worst violence was occurring in the southern province of Sindh, Bhutto's political stronghold, where at least 20 people, including a policeman, have been killed since Thursday night following her assassination hours earlier in the city of Rawalpindi.

Ignoring appeals for calm by Pakistani President Pervez Musharraf following Bhutto's slaying by a gunman, rioters armed with sticks and even firearms torched hundreds of vehicles and were roaming deserted streets across the province, according to local officials and Pakistan television reports.

"These people are uncontrollable. They are destroying everything that comes their way," said Din Mohammed, a local resident in Khirpur district, where six people were killed in violence Thursday night. "The entire city is closed and they are burning tyres everywhere on the streets."

In Larkana district, violence erupted as tens of thousands of people were descending on Bhutto's ancestral village of Garhi Khuda Baksh in hopes of attending her scheduled 2:30 pm funeral Friday.

Security forces "blocked roads leading to the village, and angry people (responded by) burning cars, motorbikes, buildings and chanting anti-government slogans," according to a Deutsche Presse-Agentur dpa reporter on the scene.

Paramilitary troops, acting under shoot-on-site orders, were patrolling streets in several cities of Sindh province, where seven more people died in unrest on Friday.

"We have deployed 16,000 troops across the province," Major Asad Ali, spokesman for the paramilitary troops, also known as the Rangers, told Aaj TV. "With this limited force, we are trying to reach the most-affected areas."

More than 400 inmates in three prisons in Thatta district were set free by hundreds of protesters who attacked and set the facilities on fire.

In the provincial capital Karachi, however, the situation was calmer Friday morning after seven people, including a policeman, were killed in overnight violence in various parts of the port city.

"The situation is much calmer today," said city police chief Azhar Ali Farooqi. "The miscreants set four police checkpoints and more than 180 vehicles on fire, and the police have arrested 40 of them."

All land routes linking Karachi with the rest of the country were blocked after Pakistan Railways officials suspended train service. Mobs had ransacked several railway stations and set a train on fire, and roads in and out of the city had already been blocked by the rioters.

But several other cities and towns remained under the control of mobs, who looted dozens of banks and ATM machines, according to local media reports.

There were also reports of low-scale unrest in the North-West Frontier, Balochistan and Punjab provinces.

A general strike was also being observed in cities across the country, with all business activity halted, and government offices and schools closed.