InsideClimate News spent Sunday covering the People's Climate March. This story was last updated at 4:45 PM.

NEW YORK—They took the A train—or the B, C, or D, or the 1 or the 2. They piled off buses and ambled across Central Park. All along the Upper West Side, from Columbus Circle at 59th for at least 30 blocks north, the crowd poured in for what organizers had billed as the biggest public demonstration ever to push for action on the climate change crisis.

By midafternoon on Sunday, it was clear that prediction had been fulfilled. The crowd was so thick, and moving so slowly, that marchers at the rear of the line were fretting that if they made it in time to their bus pick-up spots on 34th Street it would be a miracle indeed. They had more than 50 blocks to go, and there were an estimated 310,000 people in front of them.

The marchers had started to assemble in mid-morning, and all afternoon they marched, danced, chanted, jostled and cheered themselves along.

At Columbus Circle they flowed by at a rate of about 10 people per second for hour after hour, holding signs and boogying to brass bands. Thirteen blocks uptown at 72nd Street pedestrian gridlock reigned supreme, as people who had hiked across Central Park lined up, waiting to join those already packed in place.

Some stopped to buy shish kebabs and Coney Island hotdogs from a vendor wearing a Sierra Club T-shirt. He was too busy to talk, but gave his name as Abdul and said he supported the march, not just because business was booming but because "they are doing the right thing."

Up the street at the Natural History Museum, Wang Qiong, a postdoctoral student in infectious diseases at Rockefeller University, said scientists at home in China and here in the United States were "on the same wavelength" when it comes to climate change. The difference, she said, is that "here I feel people are using too much energy—in summertime, the A.C. is way too cool."

She said she'd never marched in a demonstration before.

"I just want to give support and be surrounded by other people's energy," she said.

It was an orderly, even a happy crowd. Coming up the steps from the subway, they were met by "greeters" wearing T-shirts identifying them as volunteers, ready to point uptown or downtown for out-of-towners, of whom there were many. Other volunteers kept the porta-potty queues as polite as anything at London's march—there were marches all around the world, but none rivaled this one.

Johanna Miller, advocacy director for the New York Civil Liberties Union, on hand to provide legal counsel and eyes on the authorities lest anyone became rowdy, had basically nothing to do on Sunday.

"We don't have an official position on climate change," she said of the ACLU, "but we are definitely supportive of people's right to petition their government."

By 10:30 Sunday morning the streets of Midtown were already packed. The World Wildlife Fund had handed out all its 5,000 T-shirts—which show a panda saying "Save the Humans"—long before the march began at 11:30.

Lou Leonard, WWF's vice president for climate change, said this is the first time the WWF has organized its base like this, asking its members to come out for a march or direct action of this size.

So far, he said, he'd been blown away. "I mean, look around, right?"

Organizers had been mobilizing people from New York's five boroughs and its suburbs for months, along with those who had pledged to attend from across the country. More than 1,500 organizations, large and small, helped organize the march, and more than 2,000 other events are scheduled around the country and the world.

Carolyn Murray, a coordinator overseeing 40 organizers in New York, described the effort, reaching down into community boards and churches, finding 472 neighborhood captains in the boroughs who each recruited 100 people to their teams. If they all showed up, that was 40,000 to 50,000 right there.

Murray has been an organizer for 25 years, cutting her teeth at the University of Massachusetts in the days of anti-apartheid divestment, a tactic the new climate mobilization has adopted.

"That divestment movement shaped me, and I'm glad to see us take a lesson from history and come full circle now with the climate movement, at this amazing moment," she said.

Back then, there was no such thing as social media. Today, Twitter is alive with talk of the march. Other new ways of publicity have boiled up too—like lighting up the façade of the United Nations building last night with calls for climate action.

UN Secretary General Ban Ki-moon, who has convened a leadership summit on Tuesday, had said he would join the marchers. Many other leaders promised to attend, too, from the Marshall Islands (threatened with inundation) to the United Nations Framework Convention on Climate Change (hoping for a big wave to carry its negotiators to a successful treaty negotiation over the next 15 months).

But this was not the leaders' march. It carefully chose to call itself the People's Climate March. There wasn't even be a grandstand podium for big shot speeches, although at local assembly points around the city megaphones and loudspeakers stirred up the growing crowds, just as there were sermons on the subject from pulpits around town.

One organizer sent out a blast email with the subject line: FOMO.

That stands for Fear Of Missing Out. It's a phenomenon that numerous people alluded to in one way or another—that all of sudden in the last few weeks, there had been a surge of interest.

You could see it on the street, as kids went to school to join their teachers in the march, and as couples strode by, handmade signs tucked under their arms.

One thing that was striking was the diversity of the effort.

Organizers were determined to bridge a historical gap between mainline environmental groups and people of color. One of their slogans was that to change everything, you have to include everybody. At the front of the march were labor organizations, environmental justice advocates, first nations, immigrants and envoys from the developing and poor nations, who are likely to be hit hardest by the effects of climate change.

But plenty of people from posher communities were there, too, and overall it seemed to be a predominantly white crowd.

At about 10:30 a.m., interest groups coalesced along Central Park West, a broad avenue that was closed for 30 blocks north of Columbus Circle.

Scientists, for example, gathered at the Museum of Natural History. Three hours later they were still right there, waiting to move forward.

Between 81st and 82nd on Central Park West, the Greenpeace group rallied around "Saving the Arctic." Greenpeace grassroots director Njambi Good said they had 3,000 RSVPs for the event, with buses coming from North Carolina, Washington, D.C., Maine, Chicago and elsewhere in the United States.

The head of the march was at Columbus Circle, where Broadway slices across 59th Street.

There, on the southernmost knoll of Central Park, a group of demonstrators meditated silently as the marchers walked rather briskly by that particular spot. One of the contemplatives, her eyes closed, held a sign that said "Wake up."

Mostly the march was a cacophony without a cadence. It slowed, it stalled, and then its energy bubbled to the surface in a call-and-response or a bit of familiar tune. We Shall Overcome. You Better Change Your Evil Ways. It's Getting Hot in Here (So Turn Off All Your Cars).

At 1:00 p.m., however, they marchers observed a brief moment of silence. Marchers stood still with their hands in the air, quiet, waiting for something they knew was on its way.

Up the avenues the silence crept. And then from the back of the parade, a noise like a rising surf came back, growing rapidly louder, and finally breaking over their stilled heads like the perfect wave on a summer day. It made you want to yell, and they did.

A group of people from Philadelphia showed up at the end of the line, at 47th Street and 6th Ave. They had finished the march two hours earlier, they said, but wanted to see the last marchers pass by. This is the "defining moment" of our time, they said, and they didn't want to miss anything.

Here's a video from ICN reporter Katherine Bagley on the end of the march: