A blue wave rolled towards Westminster today as thousands of people marched wearing the colour to call for action on climate change.
World leaders will meet next week in Copenhagen with the aim of securing a new deal to tackle rising temperatures.
Environmental campaigners, aid agencies, wildlife charities, religious groups and trade unions took part in The Wave, a march organised by the Stop Climate Chaos Coalition.
An open-top double decker bus led the march, decked with banners telling the Prime Minister: "Get on board Gordon. Make Copenhagen Count."
Behind it, a huge crowd chanted: "Climate justice. Now."
Among the crowd were Pip and Beryl Cartwright from Witney, Oxfordshire, who said they had not been prompted to protest since the miners' strike.
Mr Cartwright, 73, said: "It's for the future. It's not my generation that's going to have the problem to solve."
The former geography and geology teacher said he was "only too aware" of the dangers of global warming.
"We wanted to make a positive statement," he added.
"What annoys me in London and other big cities is all these offices leave their lights on all night when I try to turn mine off.
"I was brought up in the war so things like turning lights off is second nature to me."
Mrs Cartwright, 67, added: "Everybody's got to work together. Every individual as well as every group.
"We felt it was important to come here today."
Organisers estimated 40,000 people joined the march in London with 7,000 more at a similar protest in Scotland.
Juliana Smith, 49, from Surbiton, Surrey, was among those in central London.
"We are a normal family and this is our chance to say something," she said.
"We feel powerless most of the time."
She said the Government must be tough and force people to change.
"A carrot's not going to work," she argued.
Her husband Phil said: "It's the biggest challenge humanity's ever been faced with."