Mass demonstrations have taken place across the UK and worldwide to coincide with UN climate change talks in Bali.
Organisers said 10,000 protesters marched through central London
Marches were held in 50 cities globally, including London, Cardiff, Glasgow and Belfast.
In London protesters delivered a letter to Downing St calling for climate change measures to be a priority.
The rallies come midway through the summit, which is considering how to cut greenhouse gas emissions after current Kyoto Protocol targets expire in 2012.
Thousands of delegates from almost 200 nations are attending the two-week UN Framework Convention on Climate Change on the Indonesian island of Bali.
They are seeking progress towards a "Kyoto II" deal - a new global climate treaty. Talks will also focus on how to help poor nations cope in a warming world.
Organisers said 10,000 turned out for the London march and rally outside the US embassy.
The letter delivered to Prime Minister Gordon Brown said: "We feel that dealing with this threat should be the number one priority of the British government, a priority for all areas of policy and across all departments of government."
The letter also urged the government "to secure an equitable emissions treaty that is effective in preventing the catastrophic destabilisation of global climate and which minimises dangerous climate change."
Friends of the Earth director Tony Juniper said: "It is essential our politicians show the leadership required and ensure that the climate talks in Bali speed the world towards a low-carbon future and ensure the long-term security of generations to come."
He called for a strong climate change law that cuts UK emissions by 3% a year and includes emissions from international aviation and shipping, as well as annual milestones.
Indonesian Environment Minister Rachmat Witoelar, who is president of the conference, said he hoped the United States, which did not sign the 1997 Kyoto accord, would be brought on board.
Australia's Trade Minister, Simon Crean, has warned that both rich and poor nations must commit to slashing greenhouse gas emissions.
He said Australia would not sign any binding commitments until it has the results next year of a climate change report commissioned by new Prime Minister Kevin Rudd.
The London event was one of 50 worldwide
Mr Rudd signed papers leading to the ratification of the Kyoto Protocol as soon as he took office, reversing the policy of the previous conservative government.
The US opposes Kyoto because it does not include major polluters like China and India.
It is expected to resist pressure to agree binding emission targets, despite hopes among developing countries that emission cuts of 25-40% by 2020 for industrialised countries would be included in a final agreement when the conference ends.
The figures were agreed upon earlier this year by industrialised nations that signed the Kyoto Protocol.
It commits three dozen industrialised countries to cut their greenhouse gases an average of 5% below 1990 levels between next year and 2012, when it expires.