The massive youth-led strikes in cities around the world are believed to be the biggest protest over climate change in history.

Millions of young people took to the world’s cities Friday, flooding streets, blocking traffic and skipping school to take part in what is believed to be the biggest global climate protest in history.

The Youth Climate Strike drew potentially record crowds in several cities. In Australia, nearly 200,000 people protested in Melbourne and Sydney. Hundreds of thousands more—in Islamabad, Nairobi, Berlin, London, La Paz, New York, and as many as 1,500 other cities on every continent—joined in a global plea for elected leaders and governments to take action on the climate crisis.

It was “even bigger than we dared dream,” said Bill McKibben, environmentalist, author and founder of 350.org, which helped organize some of Friday’s protests. “At a minimum, 5 million people were out around the planet to show their determination to bend the curve of history. A remarkable day.”