Demonstrations are being held around the UK to demand action on climate change ahead of the Copenhagen summit.
Organisers Stop Climate Chaos want world leaders to reach a tough new deal on cutting emissions.
In Glasgow, about 12,000 protesters turned out on Saturday, with thousands more taking part in similar events in London and Belfast.
The protests come as the Met Office prepares to publish the raw figures it uses to assess man-made global warming.
Campaigners want Western nations to commit to an 80% cut in carbon emissions by 2050.
Range of groups
In London, a series of climate change events known collectively as The Wave are taking place.
They began with an ecumenical service at Westminster Central Hall, which involved both the Archbishop of Canterbury Dr Rowan Williams and Archbishop Vincent Nichols, head of the Catholic Church in England and Wales.
Religious leaders said they were taking part in The Wave because they "recognise unequivocally that there is a moral imperative to tackle the causes of global warming".
Up to 3,000 Christians from around the UK were expected to attend the service and the main protest afterwards.
At about 1200 GMT, they joined environmental campaigners, aid agencies, trade unions and organisations including the Women's Institute for a rally close to the US embassy in Grosvenor Square, before beginning their march to the Houses of Parliament.
Later, members of the Camp for Climate Action say they are planning to camp at an as-yet secret location somewhere in the capital.
In Glasgow, demonstrators marched from Bellahouston Park in the south of the city to Kelvingrove Park for a rally.
The turnout of up to 12,000 is thought to be Scotland's largest ever protest in support of action on climate change.
Ashok Sinha, from the Stop Climate Chaos coalition, said: "We will call on Gordon Brown to make Copenhagen count by committing rich countries to reduce their emissions by at least 40% in the next 10 years, finally putting the right sort of money on the table to help poor countries, and urgently start the process of decarbonising our energy supply.
"With bold leadership at home, Mr Brown can help inspire a fair, effective and binding international deal at Copenhagen."
Downing Street has said the prime minister is "unequivocal" about the scientific case for action against climate change.
He will join Barack Obama in Copenhagen next week, after the US President announced that he had changed his plans and would now attend the end of the conference.
Ahead of the summit, Climate Change Secretary Ed Miliband took part in "the first ever ministerial mass phone call" on Saturday, after inviting questions from the public via his website, Ed's Pledge.
He told the BBC: "We're going to go all out, the whole of the British government, over the next two weeks to make sure we get the most ambitious agreement we can."
Any agreement made at Copenhagen must become a legally-binding treaty "within months", he added.
160 years
The day of action comes as a row - dubbed "Climategate" - about the reliability of data from the University of East Anglia's Climatic Research Unit continues to wage.
Last month, hundreds of emails from the unit were leaked onto the internet, prompting claims that scientists manipulated data on global warming to strengthen the argument that it is man-made.
Now the Met Office has written to 188 countries for permission to publish material, dating back 160 years from more than 1,000 weather stations around the world, which it says proves climate change is caused by humans.
Its database is a main source of analysis for the UN's climate change science body, the Intergovernmental Panel on Climate Change, which joins talks next week at the long-awaited Copenhagen summit.
John Mitchell, head of climate science at the Met Office, told the BBC the evidence for man-made global warming was overwhelming - and the planned release of data would show that.
"So this is not an issue of whether we are confident or not in the figures for the trend in global warming, it's more about being open and transparent," he said.
The Met Office said it had already planned to publish the material long before "Climategate" and denied reports that government ministers had tried to block the publication.
Mr Miliband told the BBC he would be "very surprised" if there had been any wrongdoing on the part of the scientists involved in "Climategate".
"We're in a moment when the world is about to make some big political decisions," he said.
"And there will be people who don't want the world to make those big decisions and they are trying to use this in part to say somehow this is all in doubt and perhaps we should put the whole thing off.
"Well, I just think they're wrong about that."
Barbara Stocking, chief executive of Oxfam, said world leaders must do more to help those in developing countries cope with the effects of global warming.
"For poor people, climate change is not something in the future. Climate change is hitting them now," she told the BBC.
