Stand News said in a statement that its website and social media were no longer being updated and would be taken down.

It said all employees have been dismissed.

The outlet was the most prominent pro-democracy publication in Hong Kong and one of its last remaining critical voices - after the closure of the Apple Daily tabloid and the arrest of its publisher, billionaire Jimmy Lai.

Lai, 73, a fierce critic of Beijing, was sentenced in April to 14 months in prison.

In the latest crackdown, police raided Stand News' office after arresting six people on charges of conspiracy to publish a seditious publication.

Police did not identify those who had been detained.

Local media said those arrested were four former members of the Stand News board - included pop singer Denise Ho - as well as a former chief editor and the acting chief editor.

Analysis: Western nations express 'grave concern' over erosion of freedoms

More than 200 officers were involved in the search. They had a warrant to seize relevant journalistic materials under a national security law enacted last year.

Hong Kong, a former British colony, returned to Chinese rule in 1997 with the promise that a wide range of individual rights would be protected.

But authorities have cracked down on dissent, raising concerns about press freedoms and human rights.

Police previously raided the offices of the now-defunct Apple Daily newspaper, seizing boxes of materials and computer hard drives to assist in their investigation.

Lai, who is already jailed, was charged with sedition on Tuesday.

Officials defended the crackdown.

Li Kwai-wah, senior superintendent of the police National Security Department, said: "We are not targeting reporters, we are not targeting the media, we just targeted national security offenses.

"If you only report, I don't think this is a problem."