Hundreds of thousands of school students across Europe on Friday eschewed class to take part in massive international demonstrations calling on world powers to take real action to prevent climate catastrophe.

In the Swedish capital, Stockholm, crowds gathered to listen to Greta Thunberg, the 16-year-old high school student who sparked the FridaysForFuture when she decided to go on strike every Friday and sit outside the Swedish parliament to protest against insufficient action being taken against climate change.

"We are facing the greatest existential crisis in humanity has ever faced, and yet it has been ignored. You who have ignored it know who you are," Thunberg told the Stockholm rally.

The activist has become a global sensation for climate change campaign groups the world over. She was propelled to the international stage after a compelling speech at the United Nations climate summit last year.

Youth For Climate, the international platform uniting students around the world, called for a day of strike action across the globe and protests were slated to be observed in over 2,052 locations in 123 countries, according to Thunberg, a Nobel Peace Prize nominee.

In the Austrian capital, Vienna, police estimated that 10,500 high school students gathered in the city center, while similar figures were also observed in Rome, Italy's capital.

Milan boasted the largest turn out in the country, however, as an estimated 100,000 made their way through the streets of the northern city.

In Madrid, thousands of Spanish school children gathered in the central Puerta del Sol square to hold up placards demanding immediate action.

German school children and teens attended protests in around 200 different towns and cities.

Chancellor Angela Merkel and President Frank-Walter Steinmeier have shown support for the movement.

In London, the United Kingdom's capital, large crowds outside Westminster, the heart of British politics, held slogans such as: "There is no planet B" and "Make Earth Great Again" or even, "Make Earth Greta Again," to simultaneously incorporate a play on words and take a swipe at United States President Donald Trump, a climate change skeptic.

On Oct. 2018, leading climate scientists published a report saying we had just 12 years to limit global warming to a maximum of 1.5 C (34.7) above pre-industrial levels or face a climate catastrophe.

An increase of even 0.5 C could lead to more flooding, drought, heat and climate-related poverty, the UN Intergovernmental Panel on Climate Change said.