It's being called the largest mass mobilization on behalf of the planet in world history.
The founder of 350.org, Bill McKibben, predicted that hundreds of thousands would show up in New York City for today's People's Climate March.
It's being held in advance of a one-day United Nations Climate Summit on Tuesday (September 23).
"This is all a warm-up for a big negotiating session in Paris in 2015," McKibben told the New Yorker. "To the extent that we can build a large movement, we can help push these countries some. The real point of building a large movement is to provide a countervailing power to the fossil-fuel industry."
In May, McKibben set the stage for today's march in a Rolling Stone article called "A Call to Arms: An Invitation to Demand Action o Climate Change".
The Guardian has reported that people from 150 countries are participating in the event.
Participants include celebrities such as Leonardo DiCapri, Mark Ruffalo, Susan Sarandon, and Brad Pitt.
"I feel a moral obligation to speak out at this key moment in human history—it is a moment for action," DiCaprio said in a statement.
Meanwhile, Greenpeace has transported people by the busload to increase turnout for the People's Climate March.
The Vancouver version of the march begins at 1 p.m. outside the Canadian Broadcasting Corporation at 700 Hamilton Street.