The youth movement Fridays for Future, which urges global leaders to listen to climate scientists and stop global warming, had called on children around the world to join a global online school strike on Friday.
Thunberg, who is 18 years old, sparked the global climate protest movement, including large protests, in 2018 by skipping school on Fridays to protest outside the Swedish parliament.
Meanwhile, in Berlin, Fridays for Future activists turned one of Berlin's iconic bridges into a platform to tell people around the globe that "Another world is possible".
With paint made of potato flour and water and colours done on an organic basis with beetroot or turmeric powder, dozens of climate activists organised themselves to put together a huge banner across the Oberbaum Bridge, between the areas of Friedrichshain and Kreuzberg.
Activists in Hamburg painted a big banner reading "All for the 1.5 degree target", while protesters in Austria demonstrated on the streets of Vienna.