Climate activist Greta Thunberg appeared in an Oscar clip showcasing the power of documentary films.

This was followed by actor Mark Ruffalo announcing the winners in the documentary categories at this year's Oscars ceremony on Sunday night.

The Michelle and Barack Obama-produced "American Factory" was awarded the best documentary feature.

During the acceptance speech for the Netflix project, director Julia Reichert quoted Karl Marx and Friedrich Engels.

Reichert said: "Workers of the world, unite!" - a rallying cry from "The Communist Manifesto."

The documentary is set in an abandoned General Motors plant in Ohio that reopens when Chinese billionaire chairman Cao Dewang buys it and turns it into windshield factory Fuyao Glass America.

"Leaning to Skateboard in a Warzone (If You're a Girl)", on the other hand, won the Oscar for documentary short subject.

In the same category, Indian-American filmmakers Smriti Mundhra and Sami Khan's "St. Louis Superman" was competing.