The coordinated demonstrations were planned as the largest manifestation to date of the Fridays for Future movement, in which students forgo classes each week in favor of something they have said is more important: pleading for action on an issue that will affect every person on the planet, but young people most of all.

“You’re stealing our future!” a crowd of some 20,000 demonstrators chanted Friday outside the German government ministry in Berlin.

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The adults most responsible for the ravages of climate change “are going to be gone soon enough,” said 14-year-old Ashton Cassa, who was on the streets of Sydney on Friday with an estimated 30,000 other Australians. “It is up to us kids to make a difference.”

In New York, so many young protesters gathered outside city hall, chanting and pumping their fists, that they spilled into the nearby bike lane and street.

“Climate change is not a lie,” they chanted. “We won’t let our planet die.”

Camilo Budet, a 5-year-old kindergartner from Brooklyn, came with his mother. He carried a sign almost as large as he was, which read, “Stop polluting now.”

Peter Yarrow, of the 1960s folk trio folk trio Peter, Paul and Mary, led the group in a song. “Let the children lead,” he said, as the young crowd echoed him. “That’s what we need.”

In Montreal, a crowd of several thousand young people gathered at a park near McGill University before marching downtown. Some students drummed or blew air horns, and marchers of all ages carried homemade signs, most written in French.

“The future belongs to us,” said Federico Mazziotti, 25, a student at one of Quebec’s state universities.

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In San Francisco, middle and high school students from around the Bay Area filled a downtown plaza. “Ain’t no power like the power of the youth, cause the power of the youth don’t stop!” they chanted in unison.

“Everyone thinks that education comes first, and education is really important, but this is important, too,” said Kristen Khyzoma, 16, who left during her second period class Friday.

On a springlike day in Washington, groups of friends and schoolmates began assembling on the Capitol lawn about 11:30 a.m. They brought giant beach balls to bounce around the crowd and wands to blow soap bubbles. Some lounged on multicolored parachutes the organizers had set up on the grass.

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“There are people in that building who are leaving behind a mess for us, for our generation to clean up,” 13-year-old Arkadi Vidales said, gesturing to the Capitol dome. “We need to do something about it — now.”

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Despite their frustration, the protesters across the globe Friday delivered their dire messages in a largely peaceful way, but also with exasperation and a sense of urgency.

U.N. researchers have said the world has only the next dozen years to sharply curb carbon dioxide emissions and head off the more catastrophic impacts of climate change, such as rising seas that inundate cities, crop failures that spawn famines and more severe weather-related disasters that ravage communities and cause billions of dollars in damage.

But politicians remain far apart in their efforts to address global warming. President Trump has vowed to withdraw the United States from an international climate accord signed in 2015. And many countries that remain are far from meeting their pledges to reduce carbon emissions.

“Since our leaders are behaving like children, we will have to take the responsibility they should have taken long ago,” Greta Thunberg, the Swedish teen who launched the movement, told a U.N. climate gathering in December in a speech that, with equal parts clarity and audacity, rocketed her to fame.

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“I don’t want your hope,” Thunberg told the world’s elite at an economic forum in Davos a month later. “I want you to panic.”

The 16-year-old, who started protesting by herself in Stockholm, has inspired young people around the world to follow her example. Protests have been especially large in European capitals, but Friday’s demonstrations included large gatherings in nearly every corner of the globe.

“The first day I sat, I was all alone,” Thunberg has said. Now, “it’s amazing to talk to these people who are doing the same thing and fighting for the same cause . . . all around the world.”

Parents, educators and other adults also joined the throngs of student protesters Friday.

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In Washington, Adam Siegel came to the strike with his 14-year-old daughter, Leah, a student at McLean High School. “This is a form of education,” he said. “This is civil action. . . . This is citizenship.”

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The young people behind the strikes leveraged the power of social media to plan and organize the worldwide rallies, and on Friday they enthusiastically marched on the front lines, bullhorns in hand.

An estimated 150,000 people turned out in dozens of demonstrations across Australia. Somewhat smaller protests unfolded in cities across Asia. In Europe, capitals such as Berlin, Paris and London were filled with placard-wielding students who had packed into trains and subways to reach the demonstrations. Tens of thousands of people were estimated to have turned out in each of those cities.

Large groups also turned out in New York, Washington and some other U.S. cities in roughly three dozen U.S. states.

Some said the demonstrations have become so popular that to be in school on a Friday during a protest was the exception, not the norm. “The last time, only four people were left in class, and I didn't want to be one of them,” said 18-year-old Berlin resident Enrico Csonka.

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In Paris, the march started at the Place du Pantheon, the site of the French Republic’s mausoleum for its most cherished citizens. The students climbed atop bus stations, hung onto streetlights and chanted: “One, two, three degrees. It’s a crime against humanity.”

A large, boisterous crowd — chanting, “Solutions, not pollution” — marched through central London, weaving by landmarks such as Downing Street and Buckingham Palace.

Friday marked 13-year-old Alexandria Villasenor’s 14th week striking in front of the United Nations headquarters in New York. Since December, she had sat there mostly alone through rain, snow and the polar vortex.

On Friday, she wasn’t alone anymore.

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“Today, we are declaring the era of American climate change denialism over,” she declared to the morning’s crowd outside the United Nations.

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For all the urgency and fervor among the young people filling streets, real questions remain about whether their protests will spur action from policymakers.

Thunberg and others have said they were inspired by students from Parkland High School who became activists after last year’s deadly shooting at their school. But those efforts, along with a massive youth-led demonstration last year in favor of stronger gun laws, known as March for Our Lives, have not yet led to meaningful legislation.

One of Friday’s first planned demonstrations — in Christchurch, New Zealand — had to be cut short when a heavily armed gunman attacked two mosques. The massacre of at least 49 people brought together two scourges that have been the ominous background to young people’s lives worldwide: mass shootings and a warming planet.

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Many young climate advocates in the United States have said they want to see the so-called Green New Deal — or pieces of it — embraced by lawmakers. But so far, Democrats have remained divided over the ambitious proposals, and many Republicans, including President Trump, have mocked them as absurdly expensive while also questioning the scientific consensus around man-made climate change.

Still, the teens have won the backing of many leading environmental groups, including Greenpeace, the Sunrise Movement and 350.org. This month, more than 250 scientists released a letter of support for the school strikes.

In Europe, where the climate strikes have been filling plazas in capital cities for months, leaders such as German Chancellor Angela Merkel have endorsed the movement.

But other politicians have mocked it. Christian Lindner, leader of a pro-business party in Germany, urged students to stay in school, saying that “politics is for professionals.”

In Washington on Friday, the young people gathered on Capitol Hill had a message for the politicians inside: Get moving.

Standing toward the back of the crowd, 16-year-old Jerome Nkugwa spoke about how the impacts of climate change are expected to grow more severe over time. He nodded toward the Capitol.

“Something worse is coming,” Nkugwa said. “We came out to remind them to do something about it.”