Authorities in semi-autonomous Hong Kong have embarked on a sweeping crackdown to root out Beijing's critics after huge and often violent democracy protests convulsed the city in 2019.

The latest target is films.

"When considering a film as a whole and its effect on the viewers, the censor should have regard to his duties to prevent and suppress acts or activities endangering national security," states the new guidance, which is effective immediately.

The move sparked concerns that Chinese mainland style political censorship of films had now arrived in Hong Kong.

Hammer, a Norwegian national, received an Oscar nod for his documentary about Hong Kong's democracy protests "Do Not Split".

- Culture controls -

Hong Kong's Film Censorship Authority has traditionally employed a much lighter touch.

In more recent decades, slick mainland Chinese and South Korean blockbusters have come to dominate the regional film scene.

But there are growing signs authorities want to see an increase in mainland-style controls over the cultural and art scenes in Hong Kong.

In March, an award-winning documentary about Hong Kong's massive pro-democracy protests was pulled hours before its first commercial screening after days of criticism from a pro-Beijing newspaper.

Earlier this year a university cancelled a prestigious press photography exhibition that featured pictures of the 2019 protests, citing security concerns.

A government spokesperson said film censors would strike a "balance between protection of individual rights and freedoms on the one hand, and the protection of legitimate societal interests on the other".