Authorities have embarked on a sweeping crackdown to root out Beijing's critics after huge and often violent democracy protests convulsed the city two years ago.

A new China-imposed security law and an official campaign dubbed "Patriots rule Hong Kong" has since criminalised much dissent and strangled the democracy movement.

Authorities previously announced in June that the city's censorship board would check any future films for content that breached the security law.

But on Tuesday they unveiled a new hardened censorship law which would also cover any titles that had previously been given a green light.

"Any film for public exhibition, past, present and future, will need to get approval," commerce secretary Edward Yau told reporters.

Hong Kong's national security law bans anything authorities deem to be secession, subversion, terrorism or collusion with foreign forces.

Almost all those arrested under the law so far are democracy activists and a legislative brief given to reporters on Tuesday specifically mentioned recent documentaries that "glorified" or "incited" protests.

The new law must be approved by the city's legislature -- a near certainty, given it has been purged of any opposition over the last year.

The maximum sentence for showing illegal films will be increased to up to three years jail and a HK$1 million ($130,000) fine.

Titles deemed a national security risk by censors will not be able to appeal via the usual channels.

Instead they will have to launch a judicial review in Hong Kong's courts, a long and costly legal procedure.

Authorities can also revoke viewing licenses of venues that show titles deemed "contrary to the interests of national security".

The law will bring Hong Kong much closer to the Chinese mainland, where films are rigorously vetted and only a handful of Western movies or documentaries see a commercial release each year.

Hong Kong historically boasted a thriving film scene and for much of the latter half of the last century, Cantonese cinema was world-class.

The city still maintains some key studios, a handful of lauded directors and a thriving indie scene, but new political red lines are being drawn each month.

The announcement of Tuesday's new censorship law came as Hollywood star Nicole Kidman is filming an Amazon-funded series in the city based on a book about the gilded lives of the city's "expats".

Authorities allowed Kidman and her film crew to skip quarantine, sparking public anger last week.

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