The second "Global Climate Strike" was inspired by Greta Thunberg, a Swedish student in Stockholm who sparked the first protest event in March that included about 1.4 million students. Organizers expected Friday's turnout to be even greater, though official figures have yet to be counted.
Demonstrations took place Friday in more than 2,000 cities in 125 countries -- including New York City, Washington, D.C., Houston, Los Angeles, Miami, San Francisco and Seattle. Organizers said they hoped a minimum of 1.5 million people participated.
"Since March, we have witnessed an entire narrative shift around climate breakdown," Jake Woodier, of the UK Student Climate Network, told CNN. "The climate crisis has been driven right to the top of the agenda and politicians and the media are starting to at least have conversations about the need for action."
Hundreds of students marched in London's Parliament Square Friday, chanting and carrying signs urging governments to take greater actions to cut carbon emissions and fight climate change. Other British protests were held in Birmingham, Bristol, Newcastle, Cambridge, Leeds and Oxford.
"Our region led the industrial revolution," British lawmaker Liam Byrne said. "Now we have to lead the zero-carbon revolution. But we can't design the kind of radical system change we need with the old top-down politics. We need to build a grassroots movement for change."
Lawmaker Caroline Lucas said the youth protest sets an example for adults.
"Their moral authority in making a case that older generation is destroying their future can only be answered by urgent climate action," Lucas tweeted Friday. "Declaring an emergency isn't enough. We need action now."