Climate change protesters gather around the Houses of Parliament in central London on Saturday, two days before the start of the climate change conference in Copenhagen. (Johnny Green/Press Association/Associated Press) Thousands of people marched through central London on Saturday calling for a deal on climate change at next week's conference in Copenhagen.

Organizers of the Stop Climate Chaos Coalition had originally estimated that more than 10,000 people would participate in the event that began at Grosvenor Square and made its way to the Houses of Parliament on the River Thames.

But London's Metropolitan Police said there were about 20,000 people at the march.

"We wanted to make a positive statement," retired teacher Pip Cartwright, 72, from Witney, Oxfordshire said. "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 World Wildlife Federation — called the protest "The Wave," and organizers asked marchers to dress in blue.

The first stop for many in the march was the headquarters of BP, the large British-based oil company, because of its relationship with Alberta's tarsands project.

"We have to leave the tarsands oil in the ground," said one of the speakers.

The march was to climax with a mass "wave" around Parliament. Other "Wave" events were being held in Glasgow, Belfast, and Dublin. Similar demonstrations took place in Brussels and Berlin.

"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," she said.

Data from 1,000 weather stations to be released

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 leaked to the internet.

Skeptics of man-made global warming have said 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 emails was serious and would be looked at in detail.

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.

A second group of climate campaigners was planning an action for later Saturday.

Richard Bernard, a spokesman for Camp for Climate Action, said protesters planned to pitch tents somewhere in central London to spotlight the talks in Copenhagen.

He said his group intended to march with the main protest, and then head off to their so-far secret site in London.