About 50,000 demonstrators gathered in London`s Hyde Park alone, many donning animal costumes to resemble bumblebees, cows, polar bears and exotic fish, while others brandished placards bearing stark warnings.
"There is no planet B," "Our Children Need a Future" and "We Want 100 percent Clean Energy" read a selection, in scenes replicated across the world.
"This is the problem of our generation and the next," said 37-year-old Frenchwoman Katia Herault in London, who had only a Nemo costume protecting her from pouring rain and howling winds.
Also at the London event, Oscar-winners and best-selling musicians rubbed shoulders with protesters from the Pacific islands to Scandinavia.
"This is very personal to me. This is to do with my land. This is to do with our people," said 37-year-old Mikaele Maiava from the Tokelau islands, a territory of New Zealand threatened by rising waters.
Fashion designer Vivienne Westwood was also on the march from Hyde Park to the British parliament as leaders from 150 countries prepared to meet in Paris to hammer out a plan to cut emissions.
British actress Emma Thompson, who has campaigned against oil drilling in the Arctic, said she had seen the effects of climate change during a visit there last year.
"It helped me understand in a much more visceral, real way what was happening to the planet," she told AFP.
"So I`m here today to really get behind the climate summit in Paris, which is actually a really historical event."
Rocker Peter Gabriel, founder of the band Genesis, said climate change was "a serious threat" and any pact negotiated in Paris should have "real means of enforcing the talk, which I`m sure will be in abundance."
"Politicians want to get re-elected. If there`s enough of us, and this is happening all over the world, then they will respond," he said.
Human chains
The people-powered protests kicked off with marches across Australia, with 45,000 gathering in Sydney.
"There`s nothing more important that I can be doing at the moment than addressing climate change," said Kate Charlesworth, a doctor and mother.
In Copenhagen, which hosted the previous UN climate summit in 2009, some 5,000 people marched to parliament.
Four of the protesters were dressed as polar bears to promote a sculpture they will bring to Paris this week, depicting a dead bear covered in oil.
Spells of rain failed to dampen spirits in Amsterdam, where 7,000 joined a march energised by drum bands and dancers.
In true Dutch fashion, many turned out on their bikes to highlight pedal power as a means of sustainable energy.
In Paris, where a planned demo was cancelled over security concerns, activists responded by forming a human chain and placing empty shoes on a central square.
Thousands also linked hands in Brussels, urging leaders to reach a climate agreement.
"I hope that this chain shakes all the politicians in Europe and the rest of the world to forge a deep and sincere deal," said demonstrator Stephane Eelens.
In Berlin, around 15,000 marched from the central train station to Brandenburg Gate, where a stage was set up decorated with a globe clouded in black smoke.
Further south in Spain, some 20,000 people took part in the biggest climate march in Madrid`s history, Greenpeace said.
`Christmas cancelled`
Equally colourful demos took place in Rio, Sao Paulo, Bogota, Lima, Washington and New York.
In Mexico City, about 1,000 people marched to the main square, the sprawling Zocalo, carrying flowers, balloons and banners with slogans such as, "I take care of my planet."
"Although many deny it, we are getting close to a problem with climate change and we hope grassroots pressure will make a change," said Victor Lupian, a demonstrator in his 40s.
And in New York, hundreds of people shrugged off the winter cold, including a "Mother Christmas" in knee-length black boots and a "Father Christmas" with a sign reading: "CHRISTMAS CANCELLED DUE TO MELTING NORTH POLE."
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