Pakistan opposition leader Benazir Bhutto was assassinated in a suspected al Qaida attack as she left an election rally today, throwing the country into turmoil.

Ms Bhutto, twice the country's prime minister, was shot in the neck and body by a suicide bomber who then blew themselves up as she was driven from the rally in the city of Rawalpindi.

Suspicion immediately fell on resurgent Islamic militants linked to al Qaida and the Taliban who hated Ms Bhutto for her close ties to the West and support for the war on terror.

A local Taliban leader threatened to greet her return to the country from exile in October with suicide bombings.

Ms Bhutto's supporters erupted in anger and grief after her death, attacking police and burning tires and election campaign posters in several cities. At the hospital where she died, some smashed glass and wailed, chanting slogans against President Pervez Musharraf.

The death of the charismatic 54-year-old former prime minister put the campaign for the January 8 parliamentary elections into chaos and created fears of mass protests and violence across the nation, a key Western ally in the war against terrorism.

It also brought condemnation from around the world, led by Gordon Brown and George Bush.

Mr Bush demanded that those responsible be brought to justice.

"The US strongly condemns this cowardly act by murderous extremists who are trying to undermine Pakistan's democracy," he said. "Those who committed this crime must be brought to justice."

Mr Brown said she had been killed by "cowards afraid of democracy".

He said Ms Bhutto had shown "immense personal courage" and vowed to continue to fight terrorists at home and abroad.

"This is a sad day for democracy; it's a tragic hour for Pakistan."

Mr Musharraf convened an emergency meeting with his senior staff to discuss whether to postpone the election.

"This is the work of those terrorists with whom we are engaged in war," he said in a nationally televised speech.

"I want to appeal to the nation to remain peaceful and exercise restraint."

The attacker struck just minutes after Ms Bhutto addressed thousands of supporters of her Pakistan People's Party in a Rawalpindi park.

PPP official Sardar Qamar Hayyat said he was standing about 10 yards from Ms Bhutto's vehicle at the time of the attack.

"She was inside the vehicle and was coming out from the gate after addressing the rally when some of the youths started chanting slogans in her favour. Then I saw a smiling Bhutto emerging from the vehicle's roof and responding to their slogans," he said.

"Then I saw a thin, young man jumping toward her vehicle from the back and opening fire. Moments later, I saw her speeding vehicle going away," he added.

Ms Bhutto was taken to hospital and into emergency surgery. She died about an hour after the attack.

"At 6.16 p.m., she expired," said Wasif Ali Khan, a PPP member at Rawalpindi General Hospital.

"The surgeons confirmed that she has been martyred," Ms Bhutto's lawyer Babar Awan said.

Her supporters at the hospital exploded in anger, smashing the glass door at the main entrance of the emergency unit. Others burst into tears. One man with a PPP flag tied around his head was beating his chest.

Many chanted slogans against Musharraf, accusing him of complicity in her killing.

As news of her death spread, angry supporters took to the streets in the north-western city of Peshawar as well other areas, chanting slogans against Mr Musharraf.

In Rawalpindi, Ms Bhutto's supporters burned election posters from the ruling party and attacked police, who fled.

In Karachi, shop owners quickly closed their businesses as demonstrators burned tires on the roads.

Nawaz Sharif, another former premier and opposition leader, arrived at the hospital and sat silently next to Ms Bhutto's body.

"Benazir Bhutto was also my sister, and I will be with you to take the revenge for her death," he said.

"Don't feel alone. I am with you. We will take the revenge on the rulers."

He also questioned whether to hold the elections.

"I think perhaps none of us is inclined to think of the elections," he said.

"We would have to sit down and take a very serious look at the current situation together with the People's Party and see what we have to do in the coming days."

Hours earlier, four people were killed at a rally for Mr Sharif when his supporters clashed with backers of Mr Musharraf near Rawalpindi.

Ms Bhutto's death will leave a void at the top of her party, the largest political group in the country, as it heads into the parliamentary elections.

It also fuelled fears that the crucial vote could descend into violence.

Pakistan is considered a vital ally in the fight against al Qaida and other Islamic extremists including the Taliban.

Osama bin Laden and his inner circle are believed to be hiding in lawless north-west Pakistan along the border with Afghanistan.

The United States has for months been encouraging Mr Musharraf to reach an deal with the opposition, particularly Ms Bhutto, who was seen as having a wide base of support in Pakistan. Her party had been widely expected to do well in next month's elections.

Educated at Harvard and Oxford universities, she served twice as Pakistan's prime minister between 1988 and 1996. Her father, who also served as prime minister, was executed in 1979 two years after his ouster in a military coup.

She returned to Pakistan from an eight-year exile last October. On the same day, she narrowly escaped injury when her homecoming parade in Karachi was targeted in a suicide attack that killed more than 140 people.