Millions were taking to the streets in roughly 150 countries around the world on Friday for a global strike demanding world leaders gathering at a UN climate summit adopt urgent measures to avert an environmental catastrophe.

The protests kicked off in the Pacific islands — some of the nations most threatened by rising sea levels — and Australia, where social media posts showed huge demonstrations around the country, from the big coastal cities of Melbourne and Sydney to outback towns such as Alice Springs.

The worldwide strike was inspired by 16-year-old Swedish activist Greta Thunberg.

Thunberg, who soared into prominence after sailing across the Atlantic in an emissions-free yacht ahead of the summit, was in New York to headline a massive rally in Lower Manhattan ahead of next week's climate summit at the headquarters of the United Nations.

Swedish climate activist Greta Thunberg, 16, takes part in a demonstration as part of the Global Climate Strike in New York on Friday. (Shannon Stapleton /Reuters)

The co-ordinated student "strike" culminated in New York's financial centre, Wall Street, where some investors have embraced the fossil fuel industry.

Massive crowds overwhelmed the streets of lower Manhattan, letting out roars of "Save our planet," while anticipating an address by Thunberg.

"Right now, we are the ones who are making a difference. If no one else will take action, then we will," Thunberg told demonstrators in New York.

"We demand a safe future. Is that really too much to ask?" she said.

Sixteen year-old Swedish climate activist Greta Thunberg speaks to a large crowd of demonstrators at the Global Climate Strike in lower Manhattan. (Lucas Jackson/Reuters)

New York City officials estimated that 60,000 people marched there.

Global climate campaign group 350.org said Friday afternoon that organizers estimated more than four million people had taken to the streets. About 3,000 companies closed their doors in support of the climate strikes, it said.

The protests, in advance of a UN summit Monday aimed at accelerating action to tackle global warming, kicked off a week in which more than 5,800 climate change-related strikes and other events are set to take place in 163 countries, 350.org said in a statement.

Britain's Opposition Leader Jeremy Corbyn tweeted that 100,000 people joined the demonstration in London.

One hundred thousand here in London, thousands more around our country and millions across the world.<br><br>This movement will not be silenced. <br><br>I'm here with a message: Labour will meet your demands for a Green Industrial Revolution and real change.<a href="https://twitter.com/hashtag/ClimateStrike?src=hash&ref_src=twsrc%5Etfw">#ClimateStrike</a> <a href="https://t.co/JBLiLjB6t3">pic.twitter.com/JBLiLjB6t3</a> —@jeremycorbyn

"Our future on your shoulders," read one banner stretched across a street by students in Berlin.

Protesters took to the streets of Sydney, Melbourne and Brisbane calling for world leaders gathering at a UN Climate Action Summit to adopt urgent measures to stop an environmental catastrophe. 0:47

Protests extended beyond student-led demonstrations, with hundreds of Amazon workers walking out of their offices in Seattle, Wash. The employees held signs, some made from recycled Amazon boxes, that urged the company to stop dealing with oil and gas companies and to not make political contributions to people who deny climate change.

Amazon, which ships more than 10 billion items a year, vowed Thursday to cut its use of fossil fuels. It said it had ordered 100,000 electric vans to deliver packages beginning in 2021.

Employees of Google also participated in demonstrations, as about 60 gathered in downtown San Francisco and marched to join a larger, student-led climate strike in the city. One held a sign reading: "Google Do Better."

Amazon employees Preethy Thomas and Yuan Dong smile as they hold signs during a Climate Strike march and walkout at the Amazon Spheres in Seattle, Washington. (Lindsey Wasson/Reuters)

In Brazil, thousands mounted climate demonstrations of their own, largely aimed at President Jair Bolsonaro, whom they say is allowing fires to destroy the Amazon rainforest and worsen the environmental crisis. On Twitter, Brazilians tweeted #EleNao and #ForaSalles, calls to reject Bolsonaro and oust Environment Minister Ricardo Salles. Protesters criticized and mocked the two leaders with signs and costumes.

A few dozen people, mostly students, gathered on the steps of Rio de Janeiro's state assembly behind a banner reading "climate justice."

Global anxiety has focused on Brazil since August, when it was revealed fires in the Amazon have surged to their highest level since 2010. Many environmentalists blame Bolsonaro for policies they say favour development over environmental protections, and which have led to deforestation.

Salles told Reuters on Thursday that the majority of the Amazon remains intact, showing that Brazil is doing an "excellent job" to preserve the environment. He acknowledged the existence of manmade climate change, and said protesters are right to be concerned about the future.

"I have children, and I am concerned about their future, too," he said in an interview.

A person holds a placard with a drawing depicting Brazil's President Bolsonaro with horns during a rally in Rio de Janeiro, Brazil. (Pilar Olivares/Reuters)

In Europe, German Chancellor Angela Merkel unveiled a major new climate protection package thrashed out by parties in her coalition in all-night talks.

Meanwhile, on the streets of the capital, Berlin, crowds gathered at the Brandenburg Gate, where three activists stood on blocks of ice beneath a mock gallows.

"A lot of people support our movement, but we want to go a step further, because politicians decide on our future," said Janik Oswald, a German spokesperson for Fridays for Future, the school strike movement started by Thunberg in Sweden last year. "We urgently demand that something happens."

'Not at all a nuisance,' Thai official says

The UN summit brings together world leaders to discuss climate change mitigation strategies, such as transitioning to renewable energy sources from fossil fuels.

The issue is vital to low-lying Pacific islands, which have repeatedly asked wealthier nations to do more to prevent rising sea levels.

More than 200 young people in Bangkok stormed into the Environment Ministry demanding government action on climate change. (Soe Zeya Tun/Reuters)

Children in the Solomon Islands rallied on the shoreline wearing traditional grass skirts and carrying wooden shields in solidarity with the global movement.

In Thailand, more than 200 young people stormed into the Environment Ministry and dropped to the ground feigning death as they demanded government action on climate change.

"This is what will happen if we don't stop climate change now," said 21-year-old strike organizer Nanticha Ocharoenchai.

The Thai Environment Ministry's deputy permanent secretary, Adisorn Noochdumrong, supported the students.

"This is how the young people express their concerns, which we deem as a good sign and not at all a nuisance," he said.

Indian activists and students gather for the protest against climate change in New Delhi. (EPA-EFE)

In Palangka Raya, in Indonesia's Central Kalimantan province, youths carrying placards marched through heavy smog caused by forest fires.

In the eastern Indian city of Kolkata, around 25 school children handed out flyers at busy bus terminals and held placards that read "Save Our Planet. Save Our World."

"This is the only planet we have. We wanted to stand for it before we went to school for the day," one of the children said.

'Chinese youth have their own methods'

Global warming caused by heat-trapping greenhouse gases from burning fossil fuels has already led to droughts and heat waves, melting glaciers, rising sea levels and floods, scientists say.

Carbon emissions climbed to a record last year, despite a warning from the UN-backed Intergovernmental Panel on Climate Change in October that output of the gases must be slashed over the next 12 years to stabilize the climate.

No protests were authorized in China, the world's biggest source of greenhouse gas emissions, but Zheng Xiaowen of the China Youth Climate Action Network said Chinese youth would take action one way or another.

"Chinese youth have their own methods," she said. "We also pay attention to the climate, and we are also thinking deeply, interacting, taking action, and so many people are very conscientious on this issue."