Around 20,000 protesters in London, many dressed in blue, circled the Houses of Parliament where they created a human “wave” to highlight their concerns.
Dozens of organisations including the RSPB, Oxfam and Christian Aid, worked together as the Stop Climate Change coalition for the march, which was organised in an attempt to persuade politicians to take dramatic action at the Copenhagen climate change conference.
Demonstrators called on the British Government to force rich countries to cut their emissions by at least 40 per cent by 2020, provide $150 billion a year by 2020 to help the world’s poor cope with the impact of climate change, and increase the UK’s renewable energy supply.
Former BBC weather forecaster Michael Fish was among those leading the march. He said: “I’m very concerned. We must do something before the situation gets even worse and millions die and lose their homes.
“I hope this march will make a difference. It certainly shows the depth of feeling in this country about the issue.”
Ashok Sinha, the director of the Stop Climate Change coalition, said he estimated that more than 40,000 people had joined the protest, although the Metropolitan Police put the figure at around 20,000.
He added: “We’ve got so many people coming together. We’ve got faith-based groups, anti-poverty campaigners, green groups, students, community groups, women’s groups and unions.
“That diversity and plurality shows there is a great passion about this issue and is a massive sign that we want, and need, much more urgent and effective action.”
In Glasgow, demonstrators marched from Bellahouston Park in the south of the city to Kelvingrove Park for a rally.
Strathclyde Police said about 7,000 had turned out, which is believed to be Scotland's largest ever protest in support of action on climate change.
A march was also scheduled in Belfast.