Street protests demanding urgent action on climate change have been taking place around the world, with marches reported in more than 2,000 locations.
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 chief Ban Ki-moon.
"This is the planet where our subsequent generations will live," Mr Ban 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.
Organisers 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 mobilisation 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.