From the South Pacific to the edge of the Arctic Circle, students mobilized by word of mouth and social media skipped class Friday to protest what they believe are their governments鈥� failure to take thorough action against global warming.

The co-ordinated 鈥榮chool strikes,鈥� were inspired by 16-year-old Swedish activist Greta Thunberg, who began holding solitary demonstrations outside the Swedish parliament last year.

Since then, the weekly protests have snowballed from a handful of cities to hundreds, fueled by dramatic headlines about the impact of climate change during the students鈥� lifetime.

Thunberg, who was recently nominated for the Nobel Peace Prize, a rally in Stockholm that the world faces an 鈥渆xistential crisis, the biggest crisis humanity ever has faced and still it has been ignored for decades by those that have known about it.

鈥淎nd you know who you are, you that have ignored this and are most guilty of this,鈥� she said, as protesters cheered her name.

Friday鈥檚 rallies were one of the biggest international actions yet. Protests were underway or planned in cities in more than 100 countries, including Hong Kong; New Delhi; Wellington, New Zealand; and Oulu, Finland.

In Berlin, police said as many as 20,000 protesters, most of them young students, gathered in a downtown square, waving signs with slogans such as 鈥淢arch now or swim later鈥� and 鈥淐limate Protection Report Card: F,鈥� before marching through the capital鈥檚 government quarter with a stop in front of Chancellor Angela Merkel鈥檚 office.

In Poland, thousands marched in rainy Warsaw and other cities to demand a ban on the burning of coal, which is a major source of carbon dioxide. Some wore face masks as they carried banners that read 鈥淭oday鈥檚 Air Smells Like the Planet鈥檚 Last Days鈥� and 鈥淢ake Love Not CO2.鈥�

In India鈥檚 capital New Delhi, schoolchildren protested inaction on climate change and rising air pollution levels that often far exceeds World Health Organization limits.

鈥淣ow or Never鈥� was among signs brandished by enthusiastic teenagers thronging cobblestoned streets around the domed Pantheon building, which rises above the Left Bank in Paris. Several thousand students gathered peacefully around the landmark. Some targeted President Emmanuel Macron, who sees himself as the guarantor of the Paris climate accord but is criticized by activists for being too business friendly and not ambitious enough in his efforts to reduce French emissions.

About 50 students protested in South Africa鈥檚 capital, Pretoria, chanting 鈥淭here鈥檚 No Planet B.鈥� One protester held a sign reading 鈥淵ou鈥檒l Miss The Rains Down in Africa.鈥� Experts say Africa, with its population of more than 1 billion people, is expected to be hardest hit by global warming even though it contributes least to the greenhouse gas emissions that cause it.

Police in Vienna said about 10,000 students rallied in the Austrian capital, while in neighbouring Switzerland a similar number protested in the western city of Lausanne. Last month, lawmakers in the northern Swiss canton of Basel symbolically declared a 鈥渃limate emergency.鈥�

In Helsinki, police said about 3,000 students had gathered in front of Finland鈥檚 Parliament sporting placards such as: 鈥淒inosaurs thought they had time too!鈥�

A website used to co-ordinate the rallies listed events in over 2,000 cities.

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Carla Reemtsma, a 20-year-old university student who helped organize the protest in Berlin, said she鈥檚 part of about 50 WhatsApp groups devoted to discussing climate change.

鈥淎 lot happens on social media because you can reach a lot of young people very quickly and show them: look there鈥檚 lot of us,鈥� she told The Associated Press. 鈥淭here鈥檚 a very low threshold so we reach a huge number of people.鈥�

鈥淚 think that鈥檚 how we managed to get so big,鈥� said Reemtsma.

Many protesters in Berlin took aim at politicians such as the leader of Germany鈥檚 pro-business Free Democratic Party, Christian Lindner, for suggesting that complicated issues such as climate change were 鈥渁 matter for professionals鈥� not students.

Others, including Germany鈥檚 economy minister, Peter Altmaier, have urged students to stage the protests outside school hours.

By contrast, scientists have backed the protests, with thousands signing petitions in support of the students in Britain, Finland, Germany and the United States.

Volker Quaschning, a professor of engineering at Berlin鈥檚 University of Applied Sciences, said it was easy for politicians to belittle students.

鈥淭hat鈥檚 why they need our support,鈥� he said. 鈥淚f we do nothing then parts of this planet could become uninhabitable by the end of the century.鈥�

Scientists have warned for decades that current levels of greenhouse gas emissions are unsustainable, so far with little effect. In 2015, world leaders agreed in Paris to a goal of keeping the Earth鈥檚 global temperature rise by the end of the century well below 2 degrees Celsius (3.6 degrees Fahrenheit).

Yet at present, the world is on track for an increase of 4 degrees Celsius, which experts say would have far-reaching consequences for life on the planet.

In Germany, environmental groups and experts have attacked government plans to continue using coal and natural gas for decades to come.

Quaschning, who was one of more than 23,000 German-speaking scientists to sign a letter of support this week, said Germany should aim to fully 鈥渄ecarbonize鈥� by 2040. This would give less-advanced nations a bit more time to wean themselves off fossil fuels while still meeting the Paris goal globally.

鈥淭his is going to require radical measures and there isn鈥檛 the slightest sign of that happening yet,鈥� said Quaschning.

A poll published Friday by German public broadcaster ZDF found that 67 per cent of respondents backed the students鈥� protests during school hours, with 32 per cent opposed. The representative telephone poll conducted between March 12 and 14 involved 1,290 randomly selected voters. The margin of error was about 3 percentage points.

In Stockholm, Greta Thunberg predicted that students won鈥檛 let up their protests.

鈥淭here are a crisis in front of us that we have to live with, that we will have to live with for all our lives, our children, our grandchildren and all future generations,鈥� she said. 鈥淲e won鈥檛 accept that, we won鈥檛 let that happen and that鈥檚 why we go on strike. We are on strike because we do want a future, we will carry on.鈥�

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