Two Britons were among five foreign contractors killed in central Baghdad today when a suicide bomber drove a car filled with explosives into their convoy.

At least eight Iraqi bystanders were killed in the blast in the morning rush hour traffic, and three more foreign workers were seriously wounded.

The attack was the bloodiest in a wave of terror strikes designed to derail the smooth transfer of sovereignty from the US-led occupation to an interim Iraqi government at the end of the month. The Foreign Office confirmed the British deaths but withheld the contractors' names while their relatives were notified.

In a plea to his people, Iyad Allawi, the interim Prime Minister, who has pledged a crackdown on the terrible security crisis, called for patience and promised drastic new measures to curb violence.

The suicide bomber struck early in the morning as the convoy of three off-road vehicles drove past Liberty Square, close to the US-led coalition's headquarters. Witnesses said that the attacker pulled alongside the convoy and detonated his explosives.

One of the convoy vehicles was blown off the road and into a park by the blast, which punched a deep hole in the road and ripped the front off a residential building.

As crowds of shouting Iraqis pulled bloodied and dust-caked bodies from the rubble, papers detailing plans for the construction of vital electricity turbines by General Electric Europe were scattered, sticking in pools of blood and water from the firefighters' hoses.

The recent spiral of violence has targeted anyone working alongside the coalition or the new Iraqi government. Two senior Iraq officials were assassinated over the weekend, while a car bomb on Sunday killed four police and eight civilians in Baghdad.

Dr Allawi said that the attacks may be the work of Abu Musab al-Zarqawi, a Jordanian-born terrorist linked to Al Qaeda who has a $10 million reward on his head.

"The bomb that took place today targeted civilians who were trying to reconstruct or rehabilitate the electricity. Five of the experts were killed and three were injured," he said.

"This criminal action, this cowardly action, this vicious crime targeted the heads of the infrastructure of Iraq.

"The terrorists are trying to prevent the transfer of power and sovereignty on June 30."

Insurgents have frequently attacked Iraq's power grid and oil pipelines, hoping Iraqis will blame the coalition for their awful living conditions.

Luay Ahmed Mohammed, a hotel worker, had just stepped out of a taxi outside his workplace when the bomb went off. The taxi driver and one of his colleagues were killed in the car.

"We want to work in freedom," he said, his hands covered in blood. "We have this bloody mess because our borders are wide open," he said, blaming the coalition for failing to prevent foreign terrorists from entering the country. Falah Hassan al-Naqib, the Interior Minister, announced new visa requirements today in an attempt to stop the unregulated flow of foreigners into Iraq.

The bodies of the two Britons, as well as their murdered colleagues from France, America and the Philippines, were rushed by security forces to a morgue at Baghdad airport for full identification.

But many Iraqis at the scene had little sympathy for the electricity experts killed on their way to restore their power supply as the blistering heat of summer sets in.

Angry crowds chanted "Down with America," and started throwing debris as US troops moved in to back up Iraqi police at the scene.

Iraqi police fired into the area to disperse the huge crowd, and US troops tried to arrest one bystander, ignoring police warnings that he was mentally ill. When the man resisted arrest by four soldiers, one of the Americans beat him with a stick, leaving his head bloodied and infuriating the already hostile crowd.

The Americans then left as supporters of the rebel Shiite cleric Hojetoleslam Moqtatda al-Sadr gathered at the scene, chanting "Long live Sadr." The crowds climbed on top of one of the wrecked four-wheel drives, where they burned an American flag before torching the vehicle.

Emphasising the rising security threat, a second car bomb exploded southeast of Baghdad, the bomber using the same method of driving his vehicle among a convoy of police cars. Four people were killed.

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