"Time is running out," banners proclaimed in a carnival-like rally in freezing temperatures through central Montreal on Saturday, where many protesters accused the White House of blocking progress on climate change and threatening the world's future.
Elizabeth May, of the Sierra Club environmental group, said: "We will move the world ahead. We will not wait for George W Bush.
"Together we can save the climate. Together we will stop fossil fuels from destroying our future."
She was addressing a crowd of about 6000 people outside the Montreal conference centre, where 189 nations are meeting until 9 December to find ways to halt climate change.
Delegates are discussing the shape of the next phase of the UN's Kyoto Protocol climate pact, but discussions are being hampered between those who back emission reduction targets and outsiders, such as Washington, which are opposed to caps.
Organisers said similar marches were held in 30 cities from Sydney to London to urge governments to lower emissions of heat-trapping gases from burning fossil fuels in factories, power plants and cars.
"Together we can save the climate. Together we will stop fossil fuels from destroying our future"
Elizabeth May, Sierra Club environmental group
In Montreal, one man walked on stilts disguised as the grim reaper. Gordon Shepherd, a Scottish activist dressed as a polar bear, said: "The ice is melting, we're suffering the most, we can't get food."
Pierre Pettigrew, the Canadian foreign minister, and Stephane Dion, the environment minister, took part in the march and were booed by some protesters who accused them of doing too little. But the harshest criticisms were for President Bush.
In 2001, Bush pulled out of the UN's Kyoto Protocol, which binds about 40 industrial nations to cut emissions of greenhouse gases by 5% below 1990 levels by 2008-2012.
The US president has said Kyoto will stifle American economic growth and wrongly exclude poor countries from a first round of targets to 2012. Washington doubts that greenhouse gases will mean catastrophic floods, droughts and rising sea levels.
The Montreal talks seek ways to enlist the United States and poor nations such as China and India in discussing ways to combat climate change beyond 2012.
Washington has said it is not interested in joining new talks, but Canadian negotiators say they have not given up hope.
In London, thousands of protesters also marched, some blowing whistles and carrying banners, accusing Tony Blair, the British prime minister, of wavering on pledges to make deep carbon reduction targets beyond 2012. "No Blair betrayal on climate," one banner read.
Protesters urged US President
Bush to act against climate change
Caroline Lucas, a member of Britain's Green Party, said: "We're seeing greenhouse gas emissions rise under this government. We're seeing this government now not talking about targets, talking about technology, instead."
According to organisers, about 10,000 people turned out to call on the British government to tackle the threat of global warming. Police put the crowd at slightly more than 4000.
Phil Thornhill, national co-ordinator for the Campaign Against Climate Change, which organised the protest, siad: "Climate change is probably the greatest threat humanity faces. It has consequences of catastrophic proportions.
"We are demanding urgent action at a global level to deal with it. We need an international treaty to deal with that and we need to have a target to bring emissions down and keep to that. That's the only way it will work."