Swedish climate activist Greta Thunberg participated in her last School Strike for Climate to mark her graduation from high school on Friday.

The 20-year-old previously took a year-long "sabbatical" from school in 2019 to raise awareness for climate action.

"Today, I graduate from school, which means I'll no longer be able to school strike for the climate," Thunberg said on Twitter.

"This is the last school strike for me," she wrote, adding that her future protests would "technically" not be a school strike because she is no longer a student.

"We simply have no other option than to do everything we possibly can," she added. "The fight has only just begun."

The start of a movement

Thunberg began the School Strike for Climate movement — also known as Fridays for Future — when she sat down, alone, outside the Swedish parliament to demand tougher action on climate change in 2018.

She was 15 years old at the time.

The demonstration went on to spark similar protests around the world, with school students skipping class to demand more urgent climate action.

"When I started striking in 2018 I could never have expected that it would lead to anything," Thunberg said on Friday.

"Quite suddenly this was a global movement growing every day. During 2019, millions of youth striked from school for the climate, flooding the streets in over 180 countries."

She has since become a figurehead for climate action, being invited to address leaders at United Nations conferences, becoming Time magazine's youngest-ever Person of the Year in 2019, and receiving several nominations for the Nobel Peace Prize.

In March, Thunberg condemned what she called an "unprecedented betrayal" from world leaders after the publication of the latest report by the IPCC, the UN's climate advisory panel.