Protesters march from Brussels-North Station with placards as they take part in a Global Climate Strike demonstration on September 20, 2019 in Brussels, Belgium | Jack Taylor/Getty Images Climate strikes sweep the globe Youth activism is putting pressure on governments to do more about climate change.

Millions of people took to the streets in cities around the world on Friday to call for action against climate change, in what has been dubbed the largest climate protest ever.

The demonstrations — scheduled to continue for a week — were planned in over 150 countries. Protesters marched on low-lying Pacific islands under threat from rising seas, and in cities from Warsaw to Melbourne, from Nairobi to London and hundreds of other places; #GlobalClimateStrike, #FridaysForFuture, #ClimateChange were top trending topics on Twitter.

The marches come ahead of Monday's U.N. climate summit in New York, where Secretary-General António Guterres will cajole national leaders to boost their emissions reductions promises for 2030 and to commit to becoming climate neutral by 2050.

The protests, and the U.N. summit, are reactions to the grim drumbeat of scientific reports showing that global warming is getting worse, and steps taken so far fall well short of what's needed to bring it under control.

"This planet is getting hotter than a young Leonardo DiCaprio," said one sign waved in Poland. "There is no planet B," said another in the U.K. "Denial is not a policy," read a sign in Dublin.

Friday's strikes were launched last year by teen climate activist Greta Thunberg, who started holding solitary protests outside the Swedish parliament. She is due to lead the march in New York, where the school district authorized 1.1 million students to skip school.

"Its bedtime in New York ... so please share as many pictures as you can as the strikes move across Asia to Europe and Africa!" Thunberg tweeted Thursday night.

Her idea has been copied by students around the world, sparking "Fridays for Future" protests demanding governments address the climate crisis.

"We are striking in Croatia today — people of every age are invited to join the movement, because we believe that climate change is everybody's problem," Leonarda Šmigmator, a youth leader from Fridays For Future Croatia, told Friends of the Earth Europe, an NGO.

Politicians have scrambled aboard the campaign as well. Opinion polls show worry about climate change is rising to the top of voter concerns, and they're expressing those fears at the ballot box.

"Our shared planet is facing a climate emergency. The science is clear that, without urgent action, sea levels will rise further, extreme temperatures will become the norm and climate-related disasters will inflict even greater damage," said a statement issued by the mayors of Paris, New York, Los Angeles and Copenhagen. "When your house is on fire, somebody needs to sound the alarm. Young people in our cities, displaying incredible maturity and dignity are doing just that."

Frans Timmermans, nominated to be the European Commission executive vice president in charge of the European Green Deal, met Friday with youth leaders in Brussels. "To everybody out marching today: we are listening!" he tweeted.

Many labor unions are also joining in, including in France, Germany, Belgium, the Netherlands, the U.K. and Spain, jettisoning earlier skepticism about aggressive moves to combat climate change that could have a big impact on jobs and the economy.

"For the unions, it's important because everything we have been fighting for since we exist, social justice and working conditions, will be in danger with the climate crisis," said Ludovic Voet, confederal secretary of the European Trade Union Confederation.

He called on policymakers to reconcile the demands of the youth asking for more climate action with the Yellow Vests movement that rocked France last winter, which was sparked by an increase in fuel taxes.

“It’s not possible to say to someone who’s struggling to reach the end of the month, to think about the end of the world. But we feel that it’s the same fight … a social and ecological fight,” Voet said.