Around 50,000 people marched through London to Whitehall to demand that world leaders take urgent action to tackle climate change in the biggest demonstration of its kind the UK has ever seen.
Environmental campaigners from across the country and beyond were joined by charity bosses and celebrities including actress Emma Thompson, fashion designer Vivienne Westwood and singer Charlotte Church as they called on politicians to agree an ambitious new climate agreement.
Labour leader Jeremy Corbyn received a deafening applause when he sent a message on behalf of campaigners to politicians gathering for talks in Paris next week, telling them: "Do what you are sent there to do."
The march in the capital was one of 80 in the UK and almost 2,500 around the world as demonstrators issued a clarion call for action to combat global warming and shift the world towards renewable energy.
The BT Tower in central London carried a message on its infoband supporting the #go100percent clean energy campaign, which the BT-backed 100% Sport drive is part of.
A march in Paris that was called off after the terror attacks two weeks ago was carried out symbolically when thousands of pairs of shoes were placed in the Place de la Republique, including pairs from the Pope and United Nations secretary general Ban Ki-moon.
Leaders from more than 50 countries are meeting in the French capital from tomorrow for crucial UN talks to secure strong deals to curb rising temperatures.
Thousands braved gales and rain as they gathered near Hyde Park for the start of the London rally.
Addressing the crowds, Mr Corbyn said: "The issues facing the world in Paris this week are pollution, climate change, inequality, environmental refugees, war refugees and resources wars.
"If we are to make a real change in Paris, all those issues have got to be thought about and addressed."
Mr Corbyn, who was joined by fellow Labour MPs including shadow chancellor John McDonnell, added: "Those that are sitting around the tables in Paris have an enormous opportunity in front of them.
"Do not let them flake it or fail us. Make them understand that if we are to bequeath to future generations a world of flooding, a world of environmental disaster, a world where our children will not be able to live or eat properly then they will do nothing and allow the pollution of this planet to continue.
"If, however, they listen to the words of millions all around the world who are saying it is possible to reduce the level of emissions, it is possible to slow the rate of temperature change, it is possible to protect large parts of the environment of this planet, then they will have done something useful.
"Our message today is to them - 'Do what you are sent there to do'."
Political activist Westwood added: "The rotten financial system is the cause of all our problems - poverty, war, climate change. Global warming is at the tipping point. If we go past it we can't stop it. We are there right now. We have to stop it."
Actress Thompson said afterwards: "I'm excited by the fact that in Paris all of the countries in the world will be represented and that's because of movements like this happening all over the world."
As people got into the Christmas mood at the nearby Winter Wonderland attraction, hordes of passionate campaigners marched down Piccadilly waving banners, placards and flags while Radiohead star Thom Yorke played a DJ set aboard a Greenpeace float decked with polar bears.
Some represented pressure organisations such as anti-fracking groups from Lancashire, and others were from international charities like Oxfam, Action Aid and Tear Fund.
Others had come from further afield - delegates of Pacific island nations represented at the Paris talks and indigenous people from the Arctic Circle protesting that climate change is destroying their way of life.
Kath Jones, 53, a university lecturer from Folkestone in Kent, said she wanted to "stand up and be counted" and try to persuade governments to stop pandering to fossil fuel companies.
She said: "I know the way our power politics now has to take that into consideration but this - climate change - is something that cuts through all that.
"I would ask them to bring in all those people outside the formal talks, all those affected by climate change, and listen to what they have to say."
Calum Harvey, 24, from Cambridge, added: "It is the issue of our generation. If I don't do something about this then there is nothing else that is more important. Our planet underlines everything that we do and and if we can't live on it then we really are in trouble.
"Politicians need to listen to people on the marches and those on the front line who are suffering from climate change and apply their economics that they are so obsessed with to policies and areas that will actually benefit the most people."
Friends Of The Earth chief executive Craig Bennett, who attended today's march in London, said governments could not ignore the calls for action.
He said: "From Australia to Mozambique and the Philippines, as well as here in London, millions of our fellow citizens are urging our leaders to stand up to the global crisis of more floods, storms, droughts and rising seas. We know the challenge we face, we know the solutions: now is the time to act."
Later organisers said more than half a million people marched in the 2,300 events across 175 countries, the largest climate change demonstrations in history.
More than 140,000 marched in Australia and 33,000 in New Zealand, and a demonstration even took place in Sana'a, Yemen, despite bombs falling close to the start of the march.
Nuns protested in South Korea, people demonstrated in the tiny Pacific nation of the Marshall Islands and the French territory New Caledonia, and in Kenya they marched across the equator.
Emma Ruby-Sachs, campaign director of Avaaz, the activist network which organised the protests, said: "The charge from the streets for leaders to act on climate has been deafening, with record numbers turning out across the world.
"Over 570,000 people are calling with one voice for global leaders to deliver a 100% clean energy future at the Paris climate summit."
The total number is likely to rise further with big marches in Mexico and Canada taking place later today.