The protest was organised by a coalition of green groups and charities calling for action to prevent global temperatures rising more than two degrees centigrade.
Tens of thousands of people joined a climate change march in central London on Saturday calling for world leaders to agree a deal to protect the environment at their summit in Copenhagen.
The protest was organised by a coalition of green groups and charities calling for action to prevent global temperatures rising more than two degrees centigrade, seen by many scientists as the threshold for dangerous climate change.
The marchers, many wearing blue clothes and face paint, made their way towards the Houses of Parliament chanting slogans and blowing whistles, bearing placards saying 'Climate Justice Now' and 'Climate Change: The End Is Nigh'.
Organisers of the Stop Climate Chaos demonstration said more than 50,000 took part. Police put the number at 20,000.
UK prime minister Gordon Brown met a delegation from the march and told them he would push for a far-reaching agreement at the Copenhagen meeting, which starts next week.
"I have given them the reassurance that I will fight for an agreement at Copenhagen that is ambitious and one that deals with the climate change problem in the best way that we can," he said.
He dismissed global warming deniers as "anti-change, anti-science, anti-reform, almost flat-earth opponents of climate change."
Environmental sceptics say leaked emails from a British climate research institute show scientists colluded to make global warming data look more convincing.
Energy secretary Ed Miliband joined the march and told BBC television that Britain's Meteorological Office would issue data from hundreds of weather stations around the world that would rebut the doubters.
"(The Met Office) are going to release the data so that those sceptics who say there is something to hide have no place to go," Miliband said.
He said scientists were "in no doubt about the science of climate change, that it is man-made and it is happening."
Later around 200 people from the Climate Camp protest group set up 30 tents in Trafalgar Square in the centre of London, saying they planned to stay there for 48 hours.
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