The head of Hong Kong's journalists' union has been charged with obstructing a police officer in the course of their duty, amid an ongoing crackdown on critics of the government under the national security law.

"I just received a call from the police asking me to go to the Mong Kok Police Station today for them to formally file a case against me," Ronson Chan, chairman of the Hong Kong Journalists' Association (HKJA) announced via his Facebook page on Monday.

Chan arrived at the police station at 3.30 p.m. local time and left after half an hour, after being formally charged with "obstructing official duties."

He will appear at West Kowloon Magistrates' Court on Sept. 22.

Chan, former deputy assignment editor at now-defunct pro-democracy news outlet Stand News, was re-elected as HKJA chairman in June.

He has frequently spoken out against ever-diminishing press freedom in the city.

He had been planning to study journalism on a scholarship at the Reuters Institute at Oxford University, and had been scheduled to leave Hong Kong at the end of September.

It is unclear whether Chan will now be allowed to leave to take up the scholarship as planned.

Chan told reporters outside the police station he would be seeking legal advice on the matter.

"I need to seek legal advice on how to do that," he said. "[The police] also asked me if I would leave the country at the last minute."

"I told him I was planning to spend six months [overseas] and he said he would inform the court," Chan said. "It was odd that he asked my out of the blue like that, as I was waiting for them to process my bail."

Measure of declining press freedom

Chan said his arrest, which was criticized by the city's Foreign Correspondents' Club at the time, was indicative of the current state of press freedom in the city.

"I have heard a lot of things since my arrest, but I have not been able to verify them, so I won't mention them now," Chan said. "It would be ridiculous if I were unable to go to the U.K. because of this."

"I think it's plain to see the environment Hong Kong journalists are working in from this incident."

The FCC said at the time of Chan's arrest that it "supports journalists’ right to cover stories without fear of harassment or arrest."

The statement won a rebuke from China's foreign ministry, which said it constituted "interference with the rule of law" in Hong Kong, and that there was no such thing as absolute press freedom.

The HKJA said Chan was arrested after officers claimed he failed to comply with an ID check while at a venue as part of a journalistic assignment.

"Just as Ronson Chan was about to show his ID to one of the female police officers, another plainclothes officer stepped forward and yelled at him to 'cooperate'," the HKJA said in a statement at the time.

"Chan asked the policeman to show his warrant card and asked the officer to confirm his full name and department, as he could only see the surname Tan," the statement said.

"But the officer immediately issued a warning, and, within a few minutes, had Chan in handcuffs under arrest, en route back to Mong Kok police station."