Hundreds of thousands of school students worldwide walked out of their classrooms on Friday in a mass global protest against government inaction over climate change.

From Australia to Mexico, South Africa, and Portugal, they marched, brandishing signs that read, "There is no planet B," "You're destroying our future" and "If you don't act like adults, we will."

Organizers of the so-called climate strikes had called for demonstrations in over 100 countries.

Despite three decades of warnings, carbon dioxide emissions hit record levels in 2017 and again last year.

Loading the atmosphere with greenhouse gases at current rates will eventually lead to an uninhabitable planet, scientists say.

Time is running out

In Stockholm, 16-year-old Swedish activist Greta Thunberg – who inspired the protests – warned that time was running out. 

"We are living through an existential crisis that has been ignored for decades and if we do not act now it may be too late," Thunberg, who began protesting last summer outside the parliament on school days, told Swedish public television station SVT.

She has been nominated for the Nobel Peace Prize for her actions.

Other rallies were held in 100 towns around Sweden.

Teaching a lesson

Elsewhere in Europe, thousands marched through central London with banners reading, "The future is in our hands" and "We're missing lessons to teach you one."

"Education is important but climate change is more important," 14-year-old Molly Powell said.

"They're not going to stop me trying to save the planet," said 15-year-old Joe Crabtree, who had missed two exams to join the rally.

Germany saw more than 220 rallies, including in Duesseldorf, where some 2,000 schoolchildren paraded with a carnival float depicting a giant effigy of Thunberg. "The clock is ticking and time is against us!" they shouted. "We are the last generation that can fix this."

Students also packed streets and squares in Paris, Madrid, Rome, Brussels and other cities for "Fridays for Future" protests.

'Not fake news'

Montreal was one of the cities that drew the largest crowds worldwide, estimated by organizers at nearly 150,000.

In Washington, DC, some 1,500 students rallied in front of the Capitol chanting "climate action now!", while other U.S. protests were held in New York, Washington, Chicago, Portland, Oregon, and St. Paul, Minnesota.

Further south in Latin America, placards with messages such as "climate change is not 'fake news'" were seen in Buenos Aires, Argentina, while young people also took to the streets in the Chilean capital Santiago and Colombia's Medellin.

Every continent saw demonstrations.

In Wellington, Sydney and Melbourne, student protests drew tens of thousands of people.

In Delhi, one of the world's most polluted cities, 200 students took part in a colorful protest, waving ribbons, juggling and performing stunts with hoops

In Bangkok, a small group of students who protested was later invited to meet officials at the Ministry of Natural Resources and Environment in two weeks.

Over a million

Globally, more than one million marched, according to estimates by organizing groups such as the Youth For Climate movement and AFP reporters.

The Friday for Future movement said more than 300,000 young people demonstrated in Germany alone.

Politicians react

The global action drew a mixed reaction from politicians.

Germany's Economy Minister Peter Altmaier said the demonstrators should be in class while Australia's Education Minister Dan Tehan said striking was "not something that we should encourage."

But New Zealand Prime Minister Jacinda Ardern hailed the action, saying teenagers should not wait for voting age to use their voices, adding, "We hear you."

Austrian President Alexander van der Bellen, describing climate change as the biggest challenge in the history of mankind, tweeted, "You, young people, students, give me hope that we can overcome this challenge. We, adults and politicians, need to listen to you."

UN Secretary General Antonio Guterres also backed the strikes and called for world leaders to come to the Climate Action Summit in New York in September "with concrete, realistic plans" to further reduce their emissions by 2020, in order to reach a target of 45 percent lower emissions over the next decade, and to net zero by 2050.

Over 120 countries have ratified the 2015 Paris Agreement, which calls for capping global warming at "well below" two degrees Celsius, but the planet is currently on track to heat up by double that figure.

(Cover picture: A young Austrian woman shouts slogans during a climate protest outside the Hofburg palace in Vienna, Austria, March 15, 2019. )