The People’s Climate March is campaigning for curbs on carbon emissions, ahead of the UN climate summit in New York next week.
Huge demonstrations have been taking place in Australia and Europe.
In Manhattan, tens of thousands of people are at a march that is also being attended by UN Secretary General Ban Ki-moon.
“This is the planet where our subsequent generations will live,” Ban Ki-moon told reporters.
“There is no <<Plan B>>, because we do not have <<Planet B>>.”
The UN Secretary General was accompanied by primatologist Jane Goodall and the French Ecology Minister, Segolene Royal.
Organizers of the Manhattan event said they had attracted 550 busloads of marchers, billing it as the biggest protest on the issue for five years.
They said the massive mobilization was aimed at transforming climate change “from an environmental concern to an <<everybody issue>>. ”
Business leaders, environmentalists and celebrities are expected to join the demonstration.
Hollywood actor Leonardo DiCaprio also took part, having been appointed as a UN representative on climate change last week.
In Australia, organizers said up to 20,000 people had turned out in Melbourne to call on PM Tony Abbott to do more to tackle climate change.
On September 23, the UN will host a climate summit at its headquarters in New York with 125 heads of state and government – the first such gathering since the unsuccessful climate conference in Copenhagen in 2009.
Ban Ki-moon, the UN’s Secretary General, hopes leaders can make progress on a universal agreement to be signed by all nations at the end of 2015.
He said he would “link arms with those marching for climate action” to show that the UN stands “with them on the right side of this key issue for our common future.”
The New York rally is part of a global protest that includes events in 161 countries.
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