A major climate change rally brought 20,000 campaigners to London over the weekend ahead of the UN conference on the issue which starts today in Nairobi.

Saturday's I Count rally saw marchers from the Stop Climate Chaos coalition gather outside the US embassy in Grosvenor Square where they called on Washington to ratify the 1997 Kyoto Protocol that set targets for a reduction in signatory countries' carbon dioxide emissions.

The climax of the day was a massive rally in Trafalgar Square with a star-studded line-up of performers including Razorlight and KT Tunstall.

On stage Tunstall said: "I'm here today because of the situation with the environment. We are screwing it up."

The call of the day was directed at the British government which campaigners want to see help negotiate a deal at the Nairobi conference to keep global warming below two degrees centigrade.

Demonstrators also urged Britain to lead by example and propose new laws to cut Britain's CO2 levels each year.

"Tony Blair must listen to the urgent demands of more than 25,000 people in Trafalgar Square today and millions across the UK," said Stop Climate Chaos Coalition director Ashok Sinha.

"If we all come together, we can stop climate chaos. Together we are irresistible."

A UN report published on the eve of the conference meanwhile warned that Africa could be worse hit by global warming than previously feared as large stretches of the continent's coastline face the threat of flooding, more than a quarter of animal species could see their habitats disappear and the coming decades are predicted to bring lower cereal yields.

"Africa has made the lowest contribution to climate change," said Achim Steiner, the executive director of the UN Environment Programme. "It is also the least prepared to cope with the consequences ... and has the most to lose."