Young people who took time off school to protest on Friday urged adults to heed calls by climate activists for a global general strike on Sept. 20.
In Paris, Celia Benmessaoud, 15, held up a sign saying "There's Is No Planet B," and said she hoped the school strike would change the world - echoing participants around the world, from India, Turkey and Gambia to countries across Europe.
Global warming caused by heat-trapping greenhouse gases from burning fossil fuels has already led to droughts and heatwaves, melting glaciers, rising sea levels and floods, scientists say.
Carbon emissions hit a record high last year, despite a warning from the U.N.-backed Intergovernmental Panel on Climate Change in October that output of the gases will have to be slashed over the next 12 years to stabilise the climate.