The Confederation of Trade Unions said it postponed last night's screening of Inside the Red Brick Wall - a documentary on the Polytechnic University siege in November 2019 - as staff of the film office visited its office in Yau Ma Tei at 5pm yesterday.

The Office for Film, Newspaper and Article Administration demanded the union provide information about the screening sessions and who were in charge.

"They claimed that they have the right to send someone to the screening, and to monitor if we have the screening permit and to check the participants' ID cards," the pro-democracy union said on Facebook last night.

The union planned to organize five private screenings of three documentaries, including Eternal Springs in the Mountains and Taking Back the Legislature, to members of its retail union starting on April 11.

They were about clashes at Chinese University in November 2019 and the storming of the Legislative Council in July 2019.

But the screenings became news after lawmaker Holden Chow Ho-ding of the Democratic Alliance for the Betterment and Progress of Hong Kong said they were promoting terrorism.

Chow said the national security law which came into effect on June 30 states that any events which support, incite or promote terrorism and serious violent behavior are illegal.

Before government intervention, CTU chief executive Mung Siu-tat had insisted on playing the movies.

Although the documentaries are classified as category III by the Film Censorship Authority, meaning only those aged 18 and above can watch them, they are not banned.

Mung said the documentaries revealed only social issues in the city, and are completely legal for eligible persons to watch.

He said the CTU will keep holding screenings in the future, but it will choose the venues carefully to avoid disturbance.