By Andrew Restuccia, Politico

Al Gore, Leonardo DiCaprio and U.N. Secretary General Ban Ki-moon joined a larger-than-expected throng of activists, scientists, students and elected officials who took to New York City’s streets Sunday for a massive march meant to sound the alarm about climate change.

Organizers initially estimated that the march had drawn 310,000 people, then raised that estimate to nearly 400,000 — far exceeding their projections of 100,000 attendees and making the procession through midtown Manhattan by far the largest climate-related protest in history. New York police did not offer their own crowd count.

Participants waved flags, pounded on drums and carried signs that said “No More Climate Change” and “Climate Action Now,” while police blocked traffic along Central Park West from 59th Street to 86th Street.

The scene turned a bit chaotic when Gore, Moon, scientist Jane Goodall and New York Mayor Bill de Blasio joined the march around 12:45 p.m., with police, security officers and arms-linked volunteers holding back the crowd while photographers clicked away. After a moment of silence, the crowd erupted in cheers.

Others taking part in Sunday’s protest included Sens. Bernie Sanders (I-Vt.), Chuck Schumer (D-N.Y.) and Sheldon Whitehouse (D-R.I.) as well as former Ohio Democratic Rep. Dennis Kucinich.

Participants said they were trying to send a message to elected officials that tackling climate change, an issue that has often taken a back seat in Washington, should be a top priority.

“It shows we have power,” said Sierra Club Executive Director Michael Brune. “It’s a diverse coalition. It’s broad and it’s growing in strength and it’s growing in diversity. And it’s increasingly impatient at the rate of progress.”

But it wasn’t making an immediate splash on national TV — “Meet the Press” didn’t mention the march, while CNN, Fox and MSNBC were focusing on issues like the NFL, the fight against ISIL, Friday’s White House intruder and the November elections.

The march comes just two days before more than 120 world leaders and other high-ranking officials, including President Barack Obama, are slated to descend on New York City for a United Nations climate change summit. Countries are working toward reaching an international climate change accord at the end of 2015 that would go into effect in 2020.

De Blasio said he hoped this week’s events would mark a “turning point moment” for the climate cause, but conceded that that’s far from certain.

“Summits sometimes spark great change — rallies, protests sometimes spark great change. Sometimes they don’t,” de Blasio said. He added, “My sense is that the energy you’re seeing on the streets, the numbers that have amassed here and in other cities around the world suggest something bigger is going on.”

The march was organized by more than 1,500 groups and spearheaded by 350.org, the same upstart climate activist group that has turned the proposed Keystone XL pipeline into a political quagmire for President Barack Obama. Activists mounted a massive effort to spread the word and attract the public, distributed more than 1 million flyers around New York City and chartered nearly 500 buses to bring people from around the country.

Organizers said they held more than 2,000 climate-focused events in 162 countries, and Twitter’s feeds on Sunday included photos from marches in cities like London, Berlin, Rome, Rio de Janeiro and Melbourne, Australia. The Associated Press said 40,000 people attended the London protest, including musician Peter Gabriel and actress Emma Thompson.

But the New York City march was the centerpiece.

Mary Francis, carrying a sign proclaiming herself an “angry granny,” said she came to the march from Oklahoma.

“This is a problem that my generation has created,” said Francis, 72. “My parents didn’t know about this problem. But my generation knows and we have to do what we can to fix what we can.”

New York City Comptroller Scott Stringer called the protest “a message to our dysfunctional federal government that we’re not going to be pushed out of our planet.”

While several polls have painted climate change as a marginal priority for most voters, billionaire environmentalist Tom Steyer predicted Sunday that the mass demonstration will show that it’s “a first-tier political issue, that the ability to sweep this under the rug is over.”

And Obama is one person surely paying attention, 350.org co-founder Bill McKibben said before the march kicked off.

“You don’t get to be president of the United States by ignoring huge outpourings of public sentiment,” McKibben said.