NEW YORK — In one of the largest youth-led demonstrations in history, millions of people from Manhattan to Mumbai took to the streets around the globe on Friday, their chants, speeches and homemade signs delivering the same stern message to world leaders: do more to combat climate change — and do it faster.

From small island nations such as Kiribati to war-torn countries such as Afghanistan and across the United States, young people left their classrooms to demand that governments act with more urgency to wean the world off fossil fuels and cut carbon dioxide emissions.

“Oceans are rising and so are we,” read the sign that 13-year-old Martha Lickman carried through London.

“Whose future? Our future!” shouted students from Montgomery Blair High School in Silver Spring, Md., as they made their way to the U.S. Capitol.

“I hope the politicians hear us. They don’t really seem to be doing anything,” said Albe Gils, 18, who skipped school to join the crowds of protesters in front of Copenhagen’s copper-towered city hall.

Despite a monumental turnout, it’s not clear whether the demonstrations can influence the global forces contributing to climate change or compel leaders to make the choices necessary to halt the world’s warming. But transformative change is precisely what the marchers demanded — including a swift shift from fossil fuels toward clean energy, halting deforestation, protecting the world’s oceans and embracing more sustainable agriculture.

Friday’s strikes, which spanned more than 150 countries, were largely planned by teenagers and arose as a grass-roots movement. They came three days before world leaders gather at the United Nations for a much-anticipated climate summit. U.N. Secretary General António Guterres has insisted that countries bring with them promises of meaningful action, such as vowing to reach net zero emissions by 2050, cutting fossil fuel subsidies and ceasing construction of coal-fired power plants.

The summit will offer a key test of whether the world’s nations, which came together to sign the Paris climate accord in 2015, can actually muster the resolve to scale back carbon emissions as rapidly as scientists say is necessary to avoid the worst effects of climate change.

A growing amount of research suggests that young voters in democracies are increasingly frustrated with political processes, which they say have failed to address their concerns, most notably climate change.

“I have the feeling that politicians are often just [focusing on] the next vote,” said 25-year-old student Jakob Lochner, who was attending the protest in Berlin. “If you look around, there are so many people on the street; there is kind of a social tipping point.”

In Australia, where hundreds of thousands rallied in Melbourne, Sydney and other cities, inaction on climate change and environmental degradation has made young people lose “faith in our leaders and decision-makers,” according to a UNICEF report this year. Researchers examining the same phenomenon in Europe reached similar conclusions. Almost half of all young European respondents said in a recent survey that they have no trust at all in politics.

The young protesters are part of a generation increasingly vocal in demanding that leaders take climate change more seriously. The demonstrations came more than six months after hundreds of thousands of young people staged a similar coordinated effort.

In London, tens of thousands marched past 10 Downing Street and the Houses of Parliament, some holding signs reading “Winter is NOT coming” and “I’m taking time out of my lessons to teach you.”

“We’re doing our bit, eating less meat, using less plastic,” said Lickman, the 13-year-old demonstrator. “But it’s still on the government to do something.”

Protesters in climate-conscious Germany held more than 500 events to mark the global climate strike, including a large demonstration at Berlin’s Brandenburg Gate. The demonstrations come as Chancellor Angela Merkel’s government faces increasing pressure to take bold climate action following heat waves and protests dubbed Fridays for Future throughout the country.

As the demonstration swelled, Merkel announced a wide-ranging package aimed at getting Germany back on track to meet its climate targets. Berlin has pledged to cut its emissions by 55 percent from 1990 levels by 2030. The package includes more than $60 billion in investment in such areas as trains, electric vehicles and subsidies for green buildings, according to German media.

In Moscow, Arshak Makichyan, a 24-year-old violinist, staged a one-man protest after the government rejected his application to hold a group demonstration, the BBC reported. Russia, which has been hit hard by climate change, ranks as the world’s fourth-largest emitter of greenhouse gases after China, the United States and India.

In Brussels, the young and not-so-young protested with signs in English, French and Dutch.

“I am here because we want adults to act,” said Caroline Muller, 13, who has protested in the past. “It is time to do something.”

In Washington, 35-year-old Allyson Brown pulled her 5-year-old daughter out of school and headed toward the Mall to join other protesters and impart a lesson on how to push for change.

“This,” she said, “is education for today.”

Among the largest, most high-profile protests was in New York, led by 16-year-old Swedish climate activist Greta Thunberg, who has inspired the burgeoning protest movement with the solitary school strikes she undertook outside her country’s parliament beginning last summer.

Even before the strike in Manhattan officially began, Foley Square teemed with colorful signs and shouting teenagers, and the swelling crowd spilled into the surrounding streets.

“Climate change is not a lie, we won’t let our planet die,” the masses chanted.

“Our planet is not for profit!”

Youths lead climate strikes around the world

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Climate strikers protest outside the U.S. Capitol in Washington on Friday. They are part of a global movement started last fall by Swedish teen Greta Thunberg to push for action on global warming. The D.C. strikers were chanting “shame on you.” (Astrid Riecken/For The Washington Post)

Organizer Alexandria Villasenor, the 14-year-old who helped spark New York’s climate strikes when she began protesting in front of the United Nations 10 months ago, smiled as she took in the teeming crowd.

“The strike today is going to change the conversation [at next week’s U.N. climate summit],” she said. “They have to listen to us now.”

In Battery Park, a sweaty throng waited beneath the fierce midday sun in front of a stage where Thunberg would later speak.

The speeches from teenagers criticized those in power, both in government and in the corporate world.

“Their complacency is killing me,” said Isabella Fallahi, a young organizer from Indianapolis, who said Democrats and Republicans are equally culpable for the lack of climate action. “Both parties are guilty of silence. Politicians don’t simply get a medal for believing in facts.”

Kevin Patel, a fellow youth organizer from Los Angeles, leaned toward the microphone: “You are either with us in this fight or you are against us.”

Thomas Jimenez, 16, Lola Allen, 15, and Crystal Lantigua, 16, juniors at Fort Hamilton High School in Brooklyn, had raced to secure a place in front of the stage.

“Adults have a lot of opinions about our generation,” Jimenez said. “But I think we’re strong and powerful. It blows my mind to see kids our age make such a big difference.”

Behind him, a sea of handcrafted signs hinted at the sense of anger and frustration among his peers.

“You know it’s time for change when the children act like leaders and the leaders act like children,” read one.

“I’ll take my exams if you take action,” read another.

“Policymakers don’t get it,” said Yujin Kim, a 17-year-old South Korean student who had traveled to New York for a U.N. youth summit. “They’re not going to be here in 30 years. And we are. We’re going to keep speaking out until they listen.”

Organizers said more than 1,100 strikes took place across all 50 states on Friday. New York and Boston public schools granted students permission to skip school for the strikes. Numerous companies closed their doors in solidarity with the youths and encouraged employees to attend the strike.

After hours of marching and chants and speeches in New York, the sea of protesters roared as Thunberg finally took the stage.

“The eyes of the world will be upon them,” she said of the national leaders gathering next week at the U.N. summit. “They have a chance to take leadership. To prove they actually hear us.”

She paused.

“Do you think they hear us?”

The crowd screamed back: “No.”

She smiled.

“We will make them hear us,” Thunberg said, adding, “Change is coming. Whether they like it or not.”

Dennis and Lumpkin reported from Washington. Luisa Beck, Loveday Morris and Rick Noack in Berlin, Karla Adam in London, Michael Birnbaum in Copenhagen, Quentin Aries in Brussels and Timothy Bella, Dana Hedgpeth and Rebecca Tan in Washington contributed to this report.