How big was the global climate strike? 4 million people, activists estimate.

It was likely the largest climate protest in world history.

This story is part of Covering Climate Now, a global collaboration of more than 250 news outlets to strengthen coverage of the climate story.

Friday was a truly historic day for the potent new social movement committed to sounding a global alarm about the climate crisis. The Global Climate Strikes, inspired by Swedish climate activist Greta Thunberg, age 16, may end up being the largest mass protest for action on global warming in history.

The exact number of participants worldwide will be hard to get. But the event was truly global and astonishingly well organized: There were 2,500 events scheduled in over 150 countries on all seven continents.

And according to 350.org, a major environmental advocacy group and a co-organizer of today’s events, more than 4 million people worldwide took part.

“There were 40,000 people striking in France; 2,600 in Ukraine; 5,000 in South Africa; 10,000 in Turkey; 5,000 in Japan; 100,000 in London; 400,000 in Australia; 250,000 in NYC; and 1.4 million in Germany only,” 350.org writes in a press release.

(You can see photos of the strikes around the world here.)

Again, those numbers come from the event organizers so take them with a grain of salt. That said, it’s clear that Friday’s actions were enormous and spanned the globe. Kids and adults were protesting from Uganda to India, from Peru to Grenada, from Spain to Anchorage. There was even a small demonstration on the Antarctic continent.

It was inspiring to see so many young people — people who may live to see a radically different world as the next century approaches — tell the grownups of the world to heed the warnings of scientists and limit devastating warming by accelerating decarbonization in the next decade. On Monday, at the UN Climate Action Summit, we’ll find out how many were listening.

And eventually, an independent tally of participants will come. Researchers are working on their own estimates of the crowd size.

In 2017, Crowd Counting Consortium was launched by Erica Chenoweth, an expert on civil resistance at Harvard, and Jeremy Pressman, a political scientist at the University of Connecticut, as a public service project to document the scope of the Women’s March.

“Documenting the women’s march in real time, we learned how important it was for people to be seen, witnessed and counted,” Chenoweth and Pressman wrote in a Washington Post article.

They are currently collecting data on the September 20 climate strikes in the US and parallel events around the world. Eventually, they’ll have estimates of the total crowd size to share.