The Cannes film festival is set to screen a documentary on the 2019 crackdown on Hong Kong's pro-democracy movement on Friday, potentially risking diplomatic tensions with China.

A late entry slotted for the festival's penultimate day, "Revolution of our Times" chronicles the Hong Kong's government violent suppression of protests sparked by the introduction of an extradition bill allowing Hong Kong citizens to be handed over to China for prosecution.

"This film is free from self-censorship," director Kiwi Chow said in an email to AFP. "In many documentaries made about the movement, filmmakers are under immense pressure due to the current political climate of the city. I made this film completely free from any external influences," he said.

The Cannes leadership said it had been unaware of the film, the conditions of its production or the identity of the filmmaker before it was sent to festival.

"But we were enthusiastic immediately," festival director Thierry Fremaux told AFP.

"The festival is proud to present this film to show an important moment in world events. This has been Cannes's tradition and calling since 1946," he added.

For around 152 minutes, the documentary explores the protest movement, which had no clear leader, from the inside.

The documentary focuses on pro-democracy protests in 2019
The documentary focuses on pro-democracy protests in 2019 AFP / Anthony WALLACE

The involvement of ordinary citizens is prominent in the film, many of them young, their faces hidden by masks or blurred in post-production.

"I wanted to account for the development, what brought us to this point," Chow said. "I want to tell the story of Hong Kong to people who don't know about Hong Kong."

A wealth of footage of pitched battles and fighting around barricades shows Hong Kong police using extreme brutality.

Images show severe beatings of demonstrators rounded up in police traps, people pushed out of windows and suspected drownings.

Chow said he felt "anxiety and fear in my heart" making the documentary, and predicted that it will not be shown publicly in Hong Kong where police surveillance of potential protests has been beefed up.

Some of its protagonists -- who could face prosecution under a new China-sponsored security law -- are now in exile, some are in prison and others missing, Chow said.

Cannes has handled the film with unusual discretion, allowing only a handful of accredited reporters to see it before a single scheduled public screening headlined "surprise documentary" on Friday.