The officers arrested at least six media executives and editors.

Hundreds of Hong Kong national security police raided the office of online pro-democracy media outlet Stand News on Wednesday and arrested six people for suspected “seditious publications” offences, the latest in a series of crackdowns on the media.

The raid raises more concerns about press freedom in the former British colony, media advocacy groups said.

Police said in a statement they had a warrant authorising them to search and seize relevant journalistic materials.

Over 200 uniformed and plain clothes police officers have been deployed, the police informed.

Police also said they had arrested three men and three women, aged 34 to 73, for “conspiracy to publish seditious publications”. Police did not identify them but media said the six included senior members of staff.

Mr. Ronson Chan, Stand News deputy assignment editor and the head of the Hong Kong Journalists Association, was not among those arrested but said police confiscated his computer, mobile telephone, tablet device, press pass and bank records during an early morning search of his home.

Stand News has always reported news professionally, Mr. Chan told reporters.

Early on Wednesday, scores of police were seen milling about the lobby of the Stand News office in an industrial building in the Kwun Tong district.

Officers loaded about three dozen boxes of documents and other seized material onto a truck before leaving around mid-day. Reporters from other media were not allowed into the newsroom after the raid.

Steven Butler, Asia programme coordinator for the Committee to Protect Journalists, said the police action was an “open assault on Hong Kong’s already tattered press freedom”.

Stand News, set up in 2014 as a non-profit, is the most prominent remaining pro-democracy publication in Hong Kong after a national security investigation this year led to the closure of jailed tycoon Jimmy Lai’s Apple Daily tabloid.