Millions of people around the world took part in Friday's climate strike, and estimates of total crowd sizes are still rolling in — some as high as 4 million.

From New Delhi to Antarctica, protesters marched to draw attention to the climate crisis ahead of the United Nations Climate Action Summit, which kicks off Monday.

In New York City, where schools excused the city's 1.1 million students from class to participate, Mayor Bill de Blasio put preliminary crowd estimates at 60,000. Organizers, however, have pegged that number at closer to 250,000, making it the largest protest that day.

Local officials and protest organizers offered varying crowd size estimates. According to organizers, some of the other largest demonstrations took place in Berlin (270,000), London (100,000) and across Australia (about 100,000 protesting in Melbourne, organizers say).

In the U.S., big groups also turned out in San Francisco (40,000), Denver (7,500) Boston (7,000), Chicago (3,000), Portland (2,000) and Washington, D.C., among other cities.

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Greta Thunberg, the noted 16-year-old Swedish environmental activist who sparked the global movement, joined the strike in New York City. She and other organizers have estimated the worldwide crowd size at 4 million.

"Around the world today about 4 million people have been striking," Thunberg said. "This is the biggest climate strike ever in history and we all should be so proud of ourselves because we have done this together."

She's likely right — preliminary estimates suggest that Friday's strike was the largest climate protest in history.

More than 1.4 million people worldwide took part in the first global climate strike this past March, organizers estimated. Some 100,000 young people participated in the U.S. The second global strike in May likely witnessed even greater participation, organizers said.

Putting a number on turnout can be tricky, but researchers with the Crowd Funding Consortium are giving it a shot. Harvard Professor Erica Chenoweth and University of Connecticut Professor Jeremy Pressman are leading a collaborative effort to document crowd size estimates in the U.S. and worldwide.

Back in 2017, after the first Women's March, Chenoweth and Pressman launched the consortium to make crowd estimates available to academics and the public. You can see the breakdown of their estimates laid out in spreadsheets here.

"We collect data on as many U.S. protests going on as possible. We consult social media, traditional media, and, for large protests, the maps and listings publicly provided by organizers," Pressman told USA TODAY.

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The group pegged its "best guess" at total crowd size for the Women's March at more than 4 million in the U.S. alone, placing it among the largest protests in world history.

Others include the nearly 4 million people across France who marched in solidarity against terrorism following the 2015 Charlie Hebdo attack, and the 2003 worldwide protests against the war in Iraq, when somewhere between 10 and 15 million people protested, according to various estimates.

Pressman said the number of locations participating in Friday's climate strike made it difficult to put a number on crowd size, but that data was still being compiled.

A second worldwide walkout called Earth Strike is planned Sept. 27, on the anniversary of "Silent Spring," the book that kick-started the environmentalist movement.

Want to join the Crowd Funding Consortium's effort to count protesters? Submit data here.

Follow Grace Hauck on Twitter @grace_hauck.