More than 1.4 million young people around the world participated in school strikes for climate action, according to environmentalists.
Greta Thunberg, the 16-year-old Swedish student whose solo protest led to the global movement last August, said: "We have proven that it matters what you do and that no one is too small to make a difference. "
Children walked out of the schools on Friday in 2,233 cities and villages in 128 countries, with demonstrations from Australia to India, the UK and the US, according to the Fridays for the Future website. Further strikes are scheduled for April 12.
Eyal Weintraub, an 18-year-old from Argentina who participated in the protests, said: "We have reached a point in history when we have the technical capabilities to solve poverty, malnutrition, inequality and of course the greenhouse effect.
"The decisive factors for whether we are taking advantage of our potential are our activism and our international unity."
The strikes inspired by Thunberg were widely praised, with UN women say on Twitter: "She is proof that we must listen to the young generation for a sustainable future."
The executive director of Oxfam International, Winnie Byanyima, said: "Our children are leaving school and saying we have failed. This is the kind of clarity and energy that we need now."
Education ministers in the UK and Australia condemned the strikes and some commentators were critical. Madeline Grant, formerly of the think tank of the Institute of Economic Affairs, asked: "How good is it to praise children who are fundamentally wrong?"
Thunberg posted a comment on Facebook on "people who want us to go back to school". "The favorite argument here in Sweden, and everywhere else, is that it doesn't matter what we do because we are all too small to make a difference. [But] Friday was the biggest day of global climate action ever, according to 350.org, "she said.
"People keep asking me & # 39; What is the solution for the climate crisis? & # 39; They expect me to know the answer. That is more than absurd because there are no & # 39; solutions & # 39; within our current systems.
"We need a whole new way of thinking. The political system that you are [adults] is all about competition. You cheat if you can, because the only thing that matters is winning. That must come to an end.
"We must work together and share the remaining resources of this planet in an honest way. We simply pass on the words of science. Our only requirement is that you start listening to it and then playing."
• This article was amended on March 20, 2019. An earlier version referred to the next scheduled strike date on April 15. The correct date is Friday 12 April.