A Metropolitan Police spokeswoman told AFP that "about 10,000, give or take" assembled in Trafalgar Square for the culmination of a day of demonstrations involving cyclists, walkers, celebrities, politicians and "green" lobbyists.

But organisers the Stop Climate Chaos Coalition put the figure at about 14,000.

According to police, about 4,000 people gathered outside the US embassy in Grosvenor Square to call for Washington to ratify the 1997 Kyoto Protocol that set targets for a reduction in signatory countries' carbon dioxide emissions.

Ahead of a United Nations climate change conference in Nairobi, Kenya, on Monday, British Prime Minister Tony Blair's government was urged to help negotiate a deal to keep global warming to less than two degrees centigrade. Demonstrators also called for Britain to lead by example and propose new laws to cut Britain's CO2 levels further.

"The UK government can make an important case at the talks next week. There is a danger threshold: if we breach it, it could be catastrophic," said the director of the Stop Climate Chaos Coalition, Ashok Sinha.

Sinah said that the economic argument -- made in a government-commissioned report published earlier this week -- was of "fundamental importance" and could save countries billions.

The study, by former World Bank chief economist Sir Nicholas Stern, said global warming could cost the world's economies up to 20 percent of their gross domestic product.

But combatting global warming now would cost about one percent of GDP, 20 times less than the potential cost of doing nothing.

Stern also recommended a huge expansion of carbon-emissions trading networks such as that set up by the 25-nation European Union bloc, which aim to limit pollution by allowing industries to buy and sell their emission rights.

Britain, which under Blair has become one of Europe's most deregulated economies, is keen to see the EU's carbon-trading market expanded to tie in with others around the world.

The executive director of environmentalists Friends of the Earth, Tony Juniper, said the report -- which was received favourably -- should give Britain the impetus to act immediately.

"Blair needs to have the strength of leadership ... if he cuts emissions and makes changes to improve things here, he has a better prospect of convincing other countries to make changes internationally," he said.

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