London's Metropolitan Police said about 20,000 people joined the Stop Climate Chaos march, which began at Grosvenor Square and wound its way to the Parliament building on the River Thames. Organizers put the turnout at 40,000.
"We wanted to make a positive statement," said retired teacher Pip Cartwright, 72, from Witney southern England. "It's for the future. It's not my generation that's going to have the problem to solve."
The coalition - which includes groups such as Oxfam, Greenpeace, Friends of the Earth and the WWF - called the protest "The Wave," and organizers asked marchers to dress in blue. The march ended with a mass "wave" around Parliament. Thousands more people attended climate protests in Glasgow and Belfast, as well as in European cities including Brussels, Paris and Dublin.
"The U.K. government must fight for a comprehensive, fair and binding deal at Copenhagen - that is our demand today and we expect it to be fulfilled," Oxfam GB chief executive Barbara Stocking said in a statement. "They must return home with a strong, effective climate deal both for our own sakes in the U.K. and for the millions of poor people already suffering from the effects of climate change around the world."
Also on Saturday, Britain's Met Office said it would publish some of the data it uses to analyze climate change, after thousands of pieces of correspondence between some of the world's leading climate scientists were stolen from the University of East Anglia and . Skeptics of man-made global warming say the mails prove that scientists have been conspiring to hide evidence about climate change.
On Friday, the chairman of the Intergovernmental Panel on Climate Change, Rajendra Pachauri, said the issue raised by the e-mails was serious and would be looked at in detail.
Most climate scientists say their content has no bearing on the principles of climate change itself, but the leak - just before the Copenhagen summit - has been politically explosive,
Prime Minister Gordon Brown said Saturday that "the scientific evidence is strong" and it was essential for a deal on cutting greenhouse gas emissions in Copenhagen
He said those opposing action were "anti-change, anti-science, anti-reform, almost flat-Earth" groups.
Met Office spokesman Barry Gromett said data from 1,000 weather stations around the world, covering 150 years, will be released early next week. The office has written to 188 countries to ask for permission to release more data from a further 4,000 stations.
Ahead of the march through central London, Archbishop of Canterbury Rowan Williams offered prayers for negotiators in Copenhagen and urged people not to listen to those who say there's a choice between "looking after human beings and looking after the Earth."
"If we make ourselves a bit less comfortable, if we draw back from a little bit of our space and liberty so that others may have the space and comfort they need for life, thank God," Williams said.
Later, a group of climate activists pitched tents amid the fountains and statues in central London's Trafalgar Square to draw attention to the upcoming summit. Police said they were in contact with the Camp for Climate Action protesters, who planned to stay for about 48 hours.
On the Web
Camp for Climate Action: http://www.climatecamp.org.uk/
Friends of the Earth: http://www.foe.org/
Greenpeace International: http://www.greenpeace.org/international/
Oxfam International: http://www.oxfam.org/
Stop Climate Chaos: http://www.stopclimatechaos.org/
The WWF: http://www.wwf.org/