(CNN) Students worldwide skipped classes on Friday to demand that world leaders take action on a threat to their survival: Climate change.
The movement, inspired by the actions of 16-year-old Swedish environmental activist Greta Thunberg, spanned more than 100 countries and 1,500 cities, where students gathered in the streets and at their state capitols to call for action.
"Today, the tens of thousands, maybe hundreds of thousands, maybe millions of kids who are striking around the world are doing it not because we want to skip school, but because we are scared," 12-year-old Haven Coleman, who co-founded and co-directed the US Youth Climate Strike, said at a news conference in Washington, D.C.
"Climate change is the largest threat to our lives, our future and our world," she said.
Every year of these students' lives has been one of the warmest recorded. Extreme weather events, including floods, wildfires and heat waves, are becoming the new norm. And many believe that, if nothing is done to stop global warming, their generation will be left to deal with catastrophic consequences.
The global climate strikes kicked off Friday in Australia and New Zealand, where there was no shortage of chants and posters. Hoisting signs that read "Change the politics. Not the climate." and "Don't be fossil fooled," students marched in front of government buildings.
More marches followed in New Delhi and Nairobi and Tel Aviv. And later, massive crowds of students and their supporters filled the streets of European cities like Frankfurt, Rome, London and Paris, where Mayor Anne Hidalgo voiced her support.
"It is truly inspiring to see young people demanding urgent climate action," she said. "It is our responsibility as adults and political leaders, to learn from you and deliver the future you want and the future you can trust in."
In the US, marches were planned in nearly all 50 states. Footage from CNN affiliate WCCO showed a mass of students gathered on the steps of the Minnesota capitol in Saint Paul, where they chanted, "Hey hey, ho ho, fossil fuels have got to go."
Video from CNN affiliate KEYE showed a smaller group of student protesters gathered near the capitol in Austin, where they encouraged each other to continue trying to effect change. Larger crowds of students filled the streets of San Francisco after protesting at Speaker Nancy Pelosi's office, according to images from CNN affiliate KGO.
In the nation's capital, students gathered near the National Mall, calling on lawmakers to take comprehensive action. Many of them carried signs. "Change is coming," one read. "Fight now or swim l8tr," read another.
"We're all scared," 16-year-old Maddy Fernands, the national press director for the US Youth Climate Strike, told the crowd of students gathered outside of the Capitol. "But we cannot let this fear paralyze and overwhelm us."
"Hope is the only thing more powerful than fear," she added, "and hope is the only thing that is going to propel us into action."
But not everyone was on board with students skipping class to demonstrate.
Some have dismissed the students as naive or misguided. A spokesperson for UK Prime Minister Theresa May criticized student protests in February, saying that striking "increases teachers' workloads and wastes lesson time," adding that kids should be in school training to be scientists and engineers so that they can tackle the problem.
That same month, an Australian education minister warned students and teachers that they would be punished if they went on strike during school hours.
Young climate activists are hoping to spark a widespread dialogue about climate change, following in the footsteps of their peers in Parkland, Florida, who led a national conversation about gun control after a mass shooting at their school.
And they're concerned about the inaction on this front.
World leaders only have 11 more years to avoid disastrous levels of global warming, according to a 2018 report from the UN Intergovernmental Panel on Climate Change (IPCC).
If human-generated greenhouse gas emissions continue at the current rate, the planet will reach 1.5 degrees Celsius above pre-industrial levels as soon as 2030. That threshold is critical.
Global warming at that temperature would put the planet at a greater risk of events like extreme drought, wildfires, floods and food shortages for hundreds of millions of people, according to the IPCC report.
The common demand among students, although they vary country-to-country, is for the reduction of greenhouse gas emissions.
Here's what that agenda includes for kids in the US, according to the Youth Climate Strike website:
The global climate strike is an offshoot of the #FridaysForFuture movement, which has been active for months.
It began with Greta Thunberg, the 16-year-old activist, who in August 2018 started skipping school on Fridays to protest outside Sweden's parliament.
She roasted the global elite at the World Economic Forum by telling attendees they were to blame for the climate crisis. Before that, she delivered a damning speech at the United Nations' climate conference COP24, telling climate negotiators they weren't "mature enough to tell it like it is."
Her protests have inspired thousands of young people around the world. Students in countries including Australia, Thailand, Uganda and the United Kingdom have already skipped school to demand that their governments act against climate change.
"Everyone is welcome. Everyone is needed. Let's change history. And let's never stop for as long as it takes," Thunberg tweeted.