Earlier lawmaker Holden Chow Ho-ding claimed that playing the three films -- "Inside the Red Brick Wall", "Eternal Springs in the Mountains" and "Taking Back the Legislature" could violate the national security law.

The union scheduled a private screening of the film “Inside the Red Brick Wall” tonight.

At around 5 pm, six officers from the Communication Authority arrived at the office of the Confederation of Trade Unions, and required them to provide details of the showing, including the time and operator, according to the union.

They also required the union to show them the license for screening and to check the ID cards of all participants.

The union said they need more time to understand the authority of relevant departments, and decided to postpone tonight’s screening of the film.

In March, the Hong Kong Film Critics Society also cancelled the screening of the “Inside the Red Brick Wall”, after Wen Wei Po criticized the film for violating the national security law.

The society then arranged a second screening, saying the film attracted strong interest from the public.