Thousands of demonstrators demanding the withdrawal of British troops from Iraq marched through downtown London on Saturday, turning up the political heat on Prime Minister Tony Blair on the eve of his Labor Party's annual conference.

"They should never have started the war in the first place, and now it's just a mess," said Jennifer Hyde, riding a bus into the city center to join the rally, where protesters carried banners calling Blair and President Bush liars.

"We consider it to be an illegal war, and we are suffering the consequences," said Bibi Khan, an official at the London Islamic Cultural Society, joining the boisterous protest in London's famous Hyde Park. "We are upset and angry."

Polls have consistently shown weak British public support for the war in Iraq, where Britain now has 8,500 troops stationed. The issue has dogged Blair politically. He won a third term as prime minister in May, but by an extremely thin margin that most analysts traced to widespread allegations that Blair had exaggerated the threat of weapons of mass destruction in Iraq in the run-up to the war.

Public support for the war was further tested last week over a dramatic incident in the southern Iraqi city of Basra in which British soldiers were photographed jumping from their armored vehicle, their uniforms ablaze, after an angry mob attacked them with Molotov cocktails. Photos and video of the incident were played repeatedly in the British media, and photos of the burning soldiers appeared on the front pages of most major newspapers.

The soldiers escaped with relatively minor injuries, but the images ran counter to a general public perception that Basra is a far more secure part of Iraq than Baghdad and other areas of Iraq where the U.S. military is in control.

The incident also created a diplomatic and political problem for Blair. The injured soldiers were involved in a British military operation to free two British special forces troops from an Iraqi jail. The two soldiers had been arrested after allegedly shooting two Iraqi police officers who tried to detain them. One Iraqi officer was reportedly killed and the other wounded. British armored forces stormed a jail in the city and freed the two soldiers from a nearby house in an operation that infuriated residents and local officials. British officials defended the raid, saying they feared that the detained soldiers could be executed by local militiamen.

On Saturday, an Iraqi judge in Basra renewed arrest warrants for the soldiers, the Reuters news agency reported. Iraqi officials in Basra have also threatened to stop all cooperation with British troops until they repair the damaged jail and apologize for the incident. British military officials said they were investigating but defended actions taken by their troops.

"They did what they judged was appropriate at the time, and I commend them for the swift and decisive action in very difficult circumstances," Britain's defense secretary, John Reid, told reporters.

The events in Basra have hardened British public opposition to the war, said Stephan Shakespeare, a pollster and political analyst in London.

"The numbers are continuing to move against the government," he said. "There's a lot more questioning of whether we are the liberators or whether we are an army of occupation. A lot of people are starting to question things that maybe a week or two ago they were not."

Shakespeare said a public opinion poll conducted by his company to be released Sunday showed "a great pessimism" among Britons about the war. About 34 percent of Britons surveyed support the war, and 14 percent believe the situation will improve in the next 12 months, but 58 percent think it will get worse, he said. Sixty-seven percent say British troops should leave Iraq within the next year, while 27 percent want them to remain there "until the situation stabilizes," he said.

Shakespeare said Iraq would be a central topic of discussion at this week's Labor Party conference. Blair is scheduled to address the gathering Tuesday.

In a British public opinion poll, 67 percent of respondents said British troops should quit Iraq within a year.