(CNN) They flooded streets in Australia, in India, in the United Kingdom, in the United States, united in their call for the adults in charge to do something to address climate change.
There were millions of people who took part in Friday's global general strike around the world, according to 16-year-old climate activist Greta Thunberg, who helped organize the day of demonstrations.
She told a massive audience in New York, "Some people say we should study to become climate scientists or politicians, so that we can, in the future, solve the climate crisis. But by then, it will be too late. We need to do this now."
CNN could not verify the figure provided by Thunberg, who will speak Monday at the UN Climate Action Summit, but many of the marches had huge crowds.
Thunberg spoke at the end of the New York march and reiterated the urgency of the climate crisis, saying that "our house is on fire."
"It's just not the young people's house. We all live here. It affects all of us," Thunberg said.
Thunberg also called out world leaders for their inaction and decried the "lies" and "empty promises" of politicians she has met around the globe.
Thunberg said they would make those in power hear their concerns and called all those who joined her part of a "wave of change."
The crowd in New York at one point clogged streets from Foley Square down to Broadway, according to Manhattan Borough President Gale Brewer, who posted video to Twitter.
There were also big rallies in Australia where organizers said "well over" 300,000 people gathered at more than 100 cities and towns across that country. Melbourne hosted the biggest march, according to organizers, with 100,000 people turning out, while 80,000 rallied in Sydney and 30,000 in Brisbane.
Thousands of people took part in a rally just down the road from the Houses of Parliament in London.
The march was organized by UK Student Climate Network, which estimates 100 000 people were at demonstrations across London.
Organizers said this was the biggest climate change demonstration they've had so far. UK opposition party leader Jeremy Corbyn spoke at the event. He told the issue of climate change issue is "a total and absolute priority."
It wasn't just large demonstrations in population centers. There were thousands of events in more than 160 countries, Thunberg said.
Protests were held on some of the Pacific Island nations most under threat from the climate crisis -- the nation of Kiribati may be the first country to disappear under the rising sea levels of climate change -- while people turned out to march in New Zealand, Thailand and India.
"Where is my right to breathe?" read one defiant sign held up by a protestor in New Delhi, India, in a picture posted by Greenpeace India on Twitter. Meanwhile, in Thailand young protestors pretended to collapse as they demanded transformative action on the climate crisis.
In New York, Dr. Karen Stephenson, a physician, brought her two daughters to the rally.
One daughter, Mancie Bennett, 13, said she learned about the climate strike thorough the sustainability club at her school. "There's nothing more important than fighting for what you believe," she said.
It is unreasonable, she said, "to stay at home or stay at school when it's our world that's being impacted and maybe in the next 30 to 40 years it's not even going to be here for us to live on."
Her 12-year-old sister, Shani Bennett, said that young people have a responsibility to protest because "this is our life that we have to live."
The city had said its 1.1 million public school pupils could skip school for the strike on Friday without penalty but made it clear that the students did need parental consent.
As for Thunberg, she took 15 days to sail across the Atlantic -- from Plymouth, UK, to New York City. She traveled on a zero-emission sailboat to reduce the environmental impact of her journey, according to a statement from her team.
The teenager, who last August began staging weekly solo protests outside the Swedish parliament every Friday, has become the figurehead of a burgeoning movement of youth climate activists.
This week she met former US President Barack Obama and US politicians.
She has been invited to talk at the UN summit by UN Secretary-General Antonio Gutteres, who has called for heads of government to not bring speeches but plans on what he has called a climate emergency.
According to the Financial Times, leading economies such as Australia and Japan will not be invited to speak at the summit because of their continued support for coal is at odds with Gutteres' aims.
The Paris treaty, signed in 2015 by 195 nations, obliged governments to limit global temperature rises to "well below" 2 degrees Celsius and to strive for 1.5 degrees Celsius. The United States withdrew from the treaty in 2017.
In 2017, Obama lamented President Donald Trump's decision, saying in a statement that the deal was intended to "protect the world we leave to our children."
It is not just young people taking part this month. The global youth movement has asked for adults to join them this time and many have responded.
Jean-Baptiste Redde, a 62-year-old from Bourgogne, France, was striking in Paris. "I'm here today because leaders aren't taking the measures they should be taking," he told CNN. "We are all concerned. We are all part of the ecosystem, it's all linked."
Protests swept across Africa and Europe. Around 400 events were expected to take place in Germany alone. Thousands gathered in front of the Brandenburg Gate in Berlin, while Thunberg tweeted that early reports suggested over 50,000 assembled in Hamburg.
Waqas Tufail, a criminology professor, was in Leeds, England. "Climate change is killing the planet, and I'm worried about the future of my children," he told CNN.
"We need to act now, especially governments and corporations who damage the environment the most. The atmosphere is unbelievable, it's full of young people who are demanding action," Tufail added.
Sadrach Nirere, coordinator for Fridays for Future, Uganda, told CNN that it was time for young people to "rise up" and "demand urgent action."
"Uganda is facing serious frequent droughts every year, our temperatures are rising yet we are an agro-based economy that grow crops like coffee, which can thrive in such conditions," Nirere said.
"There is uncontrolled deforestation happening, the plastic bag ban is not working, there is negative political response towards climate change issues. This must stop.
"As young people, it is our time to rise up and add our voices together in demand for urgent climate action. We are rising for our future."
In Washington, students of all ages gathered late Friday morning in John Marshall Park ahead of a march to the Capitol.
Many skipped school despite being told that their absences will be considered unexcused. While some will be excused with a note from their parents, others told CNN they are willing to accept whatever consequences they receive in order to participate.
Protesters chanted, "This is what democracy looks like," while holding signs with messages such as "There is no Planet B."
A couple of hours earlier, Catholic nuns joined a related rally in nearby Silver Spring, Maryland, before heading to the Washington event.
When students arrived at the Maryland gathering, adults cheered them on, video recorded by the Sisters of Mercy shows.
"Caring for Earth is one of our five critical concerns, and we have been inspired as youth around the world have been standing up to take action," a representative for the nuns said.
Labor and humanitarian groups, environmental organizations and employees of some of the world's biggest brands are participating.
More than 1,000 Amazon employees walked out, organizers said. and Microsoft workers had also said they will join the strikes.
Microsoft Workers 4 Good tweeted earlier this month: "Microsoft workers will be joining millions of people around the world by participating in the youth-led Global Climate strike on September 20th to demand an end to the age of fossil fuels."
Outdoor clothing brand Patagonia had said it would shut down its operations Friday to allow employees to join the strike. (Stores in Italy and the Netherlands will close on September 27, and in Switzerland on September 28.)
CORRECTION: A previous version spelled Mancie Bennett's name wrong.