Around 20,000 people marched through central London on Saturday to demand action on climate change, police said, as rallies took place in other cities.
The demonstration organizers called for a tough agreement on reducing emissions at the forthcoming Copenhagen summit.
The London rally was organized by the group Stop Climate Chaos, a coalition of non-governmental organizations including Oxfam and Greenpeace.
SCC director Ashok Sinha urged British Prime Minister Gordon Brown to "make Copenhagen count by committing rich countries to reduce their emissions by at least 40 percent in the next 10 years, finally putting the right sort of money on the table to help poor countries, and urgently starting the process of decarbonizing our energy supply.
"With bold leadership at home, Mr Brown can help inspire a fair, effective and binding international deal at Copenhagen."
A spokesman for London's Metropolitan Police told AFP the demonstration had "passed off peacefully so far", with no reported arrests.
About 7,000 people marched in Glasgow, while another rally took place in Belfast. A protest was also planned for Dublin.
From the London rally, Britain's Climate Change Secretary Ed Miliband told BBC television: "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 that we can."
He added: "We are very clear that we need a treaty within months of any agreement at Copenhagen."
Meanwhile Oxfam chief executive Barbara Stocking said: "The UK government must fight for a comprehensive, fair and binding deal at Copenhagen.
"They must return home with a strong, effective climate deal both for our own sakes in the UK and for the millions of poor people already suffering from the effects of climate change around the world."
Earlier, at a tie-in ecumenical service at Westminster Central Hall, Archbishop of Canterbury Rowan Williams, the leader of the world's Anglicans, said: "We are part of a great interwoven system of life.
"Don't please listen to those that say there is some kind of choice to be made between looking after human beings and looking after the Earth."
Meanwhile the Archbishop of Westminster, the Most Reverend Vincent Nichols, the leader of England's Catholics, said: "Only when we are prepared to change the way we live will politicians be able to achieve the change that we want to see.
"Live simply so others may simply live."