NEW YORK – A boisterous crowd of at least 200,000 people turned out to chant and march in Manhattan on Friday, joining hundreds of thousands - possibly millions - of protesters from Australia to Thailand to London in Global Climate Strike rallies.

While supporters of all ages turned out, the day was billed as a walkout by high school students to call on world leaders to step up their efforts against climate change, carbon emissions and other environmental issues.

Greta Thunberg, the noted 16-year-old Swedish environmental activist whose efforts have been raising environmental consciousness around the globe, spoke to a crowd of tens of thousands in New York City's Battery Park.

"Around the world today about 4 million people have been striking," Thunberg said. "This is the biggest climate strike ever in history and we all should be so proud of ourselves because we have done this together."

New York City schools excused the city's 1.1 million students from class to participate.

"Climate change is worse than homework," read one homemade sign among the crowd marching from Foley Square to City Hall, and another said, "Stop denying the Earth is frying." At one point, the crowd, gathered on a breezy, sunny afternoon outside federal courthouses, chanted: “That’s bulls--t, get off it, our planet’s not for profit!”

"We are not in school today, and we have some adults who are not at work today either," Thunberg told the crowd. "And why? Because this is an emergency. Our house is on fire."

Katie Elder, 19, from Milwaukee, was in New York, where she is taking two gap years between high school and college to work for Future Coalition, a network of youth organizations. She called the turnout "amazing" that was turning into a "really, really powerful day."

Pig-tailed, nine- year-old Sophia of Queens, New York, who was carrying a hand-drawn "We speak for the trees"sign, said she'd begged her mother to let her skip school: "Mommy, can we pleeeease go to the climate march?" She said she already bugs her not to use plastic plates or forks, and her mom agreed they could go together, along with her sister.

The global protests were timed to begin a week of activism at the United Nations, including a Youth Climate Summit on Saturday and a U.N. Climate Action Summit on Monday. A second worldwide walkout called Earth Strike is planned for Sept. 27.

'It's our future that's at stake':US students plan to skip school Friday to fight climate 'emergency'

In Washington, D.C., several thousand young people marched to the Capitol building carrying signs reading “There is no Planet B” and “This can’t wait until I finish high school.”

“Basically our Earth is dying and if we don’t do something about it, we die,” said A.J. Conermann, a 15-year-old sophomore. “I want to grow up. I want to have a future.”

And in Chicago, crowds grew as the young people led the march through the streets to Federal Plaza. At the front of the marches stood Isabella Johnson, who told USA TODAY that her goal is to pressure politicians to combat the climate crisis, particularly by passing a proposed bill in the Illinois legislation for a Clean Energy Jobs Act.

"I want this country's leaders to realize that the youth will not back down, we will strike and strike and strike until they take action," she said. "And if they refuse to take action that would ensure my generation a healthy future, then we will vote them out of office first chance we get."

Protests around the world

As rallies began in the Pacific at sunrise and circled the globe, organizers estimated that more than 300,000 demonstrators turned out on Australian streets, including Australia's largest city, Sydney, and the capital, Canberra.

The demonstrators called on Australia, which is the world’s largest exporter of coal and liquid natural gas, to take more drastic action to reduce greenhouse gas emissions.

Australia’s acting prime minister, however, called such climate rallies “just a disruption” that should have been held on a weekend to avoid inconveniencing communities.

Organizers said protesters would be turning out in 156 countries from such disparate locations as Nepal, Senegal, Quebec, Rome, Kyrgyzstan, Sweden, Bolivia and Peru:

• In New Delhi, one of the world’s most polluted cities, dozens of students and environmental activists chanted “We want climate action” and “I want to breathe clean” at a rally outside the Ministry of Housing and Urban Affairs.

• In Bangkok, hundreds of people marched in Thailand’s capital and staged a “die-in” outside the Ministry of Natural Resources to demand the government declare a climate emergency, ban coal energy by 2025 and completely replace fossil fuel energy with renewable energy by 2040.

• In Poland, which is heavily reliant on coal, thousands of students turned out to demand more efforts to fight global warning. To mark the day, Poland’s president Andrzej Duda and first lady Agata Kornhauser-Duda helped remove trash from a forest.

• In Prague, protesters gathered in the Czech Republic's downtown Old Town Square, waving numerous banners that read “More love, less coal,” “Science, not silence,” or “Why should we go to universities when they don’t listen to the educated?” before marching through the city.

• In Kabul, the capital of war-ravaged Afghanistan, a young generation, worried that if war doesn’t kill them climate change will, took part in the global climate strike. About 100 young people, with several women in the front carrying a banner emblazoned with “Fridays for future", marched through central Kabul. They followed behind an armored personnel carrier deployed for their protection.

• In Antarctica, a group of nine researchers took part in the protest, despite being thousands of miles away in the snowy tundra. Kim Bernard, an ecologist and biological oceanographer, shared a photo on Twitter.

"It's time to rise before it's too late. We support you, climate strikers," Bernard wrote. "With love and hope from Antarctica."

Climate and change:As temperatures rise, expect public health problems to soar

•Thousands of school students and adults gathered outside the British Parliament in London to demand “climate justice.” There were also rallies in U.K. cities including Birmingham, Glasgow and Belfast. Some demonstrators held home-made placards with slogans including “Don’t be a fossil fool.”

The British government said it endorsed the protesters’ message, but didn’t condone skipping school.

More around the US

In Chicago, ahead of the main march, several labor groups rallied outside Amazon's downtown offices in support of Amazon workers who planned to walk off the job to demand Amazon take action against climate change. Employees at the Amazon headquarters in Seattle also planned to walk out Friday.

Dozens of protesters, led by Warehouse Workers For Justice and Sunrise Movement Chicago, held signs and chanted “Fossil fuels have got to go” and “Amazon’s a big polluter, do your part to save our future.” They also planned to join the city’s larger climate strike.

The protest, which was planned days ago, comes one day after Amazon CEO Jeff Bezos announced a new plan for the company to tackle climate change.

Though billed as a walkout, many public schools backed the effort as a civic exercise and learning exercise.

Chicago Public Schools said it sought to "respect and support our students’ desire to voice their opinions and participate in the wider conversations taking place about important social issues."

CPS allowed students to join the march but said they would receive an unexcused absence if they didn’t return to class afterward, according to a letter sent to principals earlier this week.

Payton Strobel stood on the steps of the Iowa State Capitol late Friday morning looking out at a tie-dyed sea of climate activism.

Strobel, a 14-year-old-freshman at Waukee Prarieview School, wore a black T-shirt as she held a microphone and summarized the feelings of student climate activists in the crowd.

"Our generation is the last, best hope," Strobel said.

More than 200 scientists from 38 Iowa colleges and universities signed a report released Thursday that said Iowa will have more than 67 calendar days per year above 90 degrees by 2050, compared with 23 days in recent decades.

Isabella Cook of West Des Moines told the crowd that strong thunder and lightning from a recent thunderstorm roused her into climate activism. Like others, Cook wants businesses and politicians to find alternatives to plastic waste that harms the environment.

"We will declare a future for ourselves," Cook told the crowd.

On the steps of the Tennessee state Capitol, Nick Clancy, 17, said climate change should be a top priority due to the issues it is causing. He said mass immigrations, water shortages and other world events should be top of mind. But he also looked toward what happens next.

"This is for everybody," said Clancy, a University School of Nashville student. "If we can solve climate change or dampen the effects then it will go toward solving other problems.

In the middle of the school day, 70 high schoolers in Greenville, South Carolina carried hand-painted signs as they walked down Main Street to City Hall.

"People still think that it’s a generation or two down the road, but no, this is just happening now," said Wylder Voegele, a 17-year-old student. "It’s scary."

Read this:As Earth faces climate catastrophe, US set to open nearly 200 power plants

Elsewhere in the U.S., individual strikes were being organized by young people in their own towns and cities, similar to last year's national school walkouts aimed at combating gun violence.

In San Francisco, organizers said their march would start at the local office of Speaker of the House Nancy Pelosi, who represents the area.

Consider this:There are 3 billion fewer birds in North America than there were in 1970

It’s not just students who are protesting. Susan Sattell, 59, a mother and member of participating organization Extinction Rebellion, planned to strike Friday. She said she joined the group after reading a United Nations report on species extinction this past May.

“I overheard my daughter Emily and her friend talking about the report. They were counting, and her friend said ‘Wow, we’ll be 24 when extinction starts.’ And my daughter said, ‘that’s pretty old, but I want to live longer than that,’” Sattell said.

Inspired by Greta Thunberg

The face of the worldwide effort is Thunberg, who came to New York on a solar-powered sailboat to attend the strike in New York City and then the summit.

While Thunberg is not the organizer, she is a major motivating force. She gave the global movement a push starting in August 2018 when she began skipping school on Fridays to stand outside the Swedish parliament holding a sign protesting inaction on climate change.

You may like:6 things to know about climate change activist Greta Thunberg

She has also met with Pope Francis who, she said, expressed support for the climate protests and has addressed the European Parliament.

In a worldwide walkout in March, Thunberg said over 1 million people took part in 125 countries, citing numbers from climate action group 350.org.

Thunberg was in Washington to demonstrate in front of the White House and to testify before Congress. She also grabbed a meeting with former president Barack Obama, who ended the session with a fist bump.

Thunberg told the Associated Press on Friday that the large turnout worldwide was "a victory."

“I would never have predicted or believed that this was going to happen someday and so fast,” she said.

Thunberg added that it was now up to world leaders to take action. She said if they don’t, they should “feel ashamed.”

After speaking, Thunberg bustled to the New York City subway, surrounded by volunteer students protecting her as she took the stairs down to the Red line trains. Teens shrieked like they were seeing a rock star.

“She’s so amazing! I saw that little pink dress and I couldnt believe it was her! Greta! No one’s ever taking the escalator again!” said Jackson Dean of Palm Springs, a freshman theater Student at New York University.

Contributing: Morgan Hines in New York; Lucas Grundmeier in Iowa; Jason Gonzales in Tennessee; Zoe Nicholson in South Carolina.