According to organisers, some 10,000 people turned out to call on the British government to tackle the threat of global warming, although police put the crowd at slightly over 4,000.

The protest coincided with others in 32 countries worldwide ahead of a critical United Nations conference of about 190 nations on climate change in Montreal, Canada.

Britain's Environment Secretary Margaret Beckett and her junior, Elliott Morley, will represent Britain at the talks, which start on December 9.

"Climate change is probably the greatest threat humanity faces. It has consequences of catastrophic proportions," said Phil Thornhill, national co-ordinator for protest organisers the Campaign Against Climate Change.

"We are demanding urgent action at a global level to deal with it. We need an international treaty to deal with that and we need to have a target to bring emissions down and keep to that. That's the only way it will work."

The demonstration, which ended outside the US Embassy, saw many bang drums and blow whistles as well as chant anti-political slogans.

Some carried banners saying "Bush, Blair, Beckett: climate criminals".

The Montreal talks follow on from the Kyoto Protocol, which was signed in 1997 and called for a 30 percent cut in carbon emissions by 2020.

A letter handed in to British Prime Minister Tony Blair at Downing Street demanded his government reaffirm its commitment with legally binding targets on emission reductions.

Blair has pledged to reduce greenhouse gas emissions by 20 percent of 1990 levels by 2010.

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