National security police visited Wong’s home and his Kowloon Bay office and issued a letter to him, asking him to remove the online posts by a certain deadline, the source said, adding that no arrests were made in the operation and an investigation was under way.

Another source said the posts were related to entries in a flag design contest organised by Passion Times in 2016. He said the Hong Kong Flag Design Competition attracted 63 entries and more than 2,000 people voted online to select the winner.

He said organising the competition was suspected of inciting Hong Kong independence and could constitute the offence of secession under the national security law.

“After enactment of the national security law, Passion Times continued to keep the posts of the designs in the competition which is a challenge to the law,” the source said, adding police then asked the online news outlet to delete all the related posts.

Wong, 42, confirmed on his Facebook page that national security police had been to his home and office in the morning.

He said he received a letter from the National Security Department that asked him to remove posts containing “sensitive” content.

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“No one has been arrested. We are safe and free to move around,” Wong wrote.

“Staying in Hong Kong today and continuing to work in the media, the risk is getting higher day by day. But I’m still in Hong Kong. I still speak on the microphone every day and I even feel a little weird.

“But we will keep working hard! Before success, we must never give up on our dreams!”

Wong is co-founder of the now-defunct radical opposition group Civic Passion. Known for advocating Hong Kong localism, the group disbanded in September last year, following the unseating of its chairman Cheng Chung-tai from the Legislative Council and the resignation of two district councillors.

Civic Passion was one of several opposition-leaning groups to break up following Beijing’s imposition of national security legislation on Hong Kong in June 2020 that outlaws subversion, secession, terrorism and colluding with foreign forces.

In August last year, Civil Human Rights Front, an umbrella group behind many of Hong Kong’s largest protests, and the city’s biggest educators’ organisation, the Professional Teachers’ Union, announced they were disbanding amid a police investigation into their activities.

Civic Passion was co-founded by Wong in 2012 and adopted a radical stance towards Beijing. It held strong localist views and espoused a “militant” style of protesting, opposing the involvement of the central government in the city’s affairs.

Meanwhile, independent publisher Hillway Press, founded by political activist and author Raymond Yeung Tsz-chun, has revealed that it was rejected from taking part in this year’s Hong Kong Book Fair, to be held in July.

In a post on its Facebook page, the publisher said fair organiser the Trade Development Council (TDC) had notified it on Monday that its application for taking part in the annual event this year was not approved.

“There was no reason given,” the post said. “We rang the TDC for inquiries on May 17 and they said no additional information could be supplied.”

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The TDC on Wednesday declined to comment on the case of Hillway Press, which took part in the Book Fair in 2019 and 2021.

It said: “In organising any event, it is not uncommon that some applications may not be successful. We do not comment on individual cases.”

Yeung, a former teacher, said he was “shocked” by the decision.

“We had followed their procedures and submitted applications and other supporting documents since January. Everything seemed smooth and there was no sign the TDC was unhappy and wanted to reject our application,” he said.

“Actually, we were invited to go to select booths on May 6 but were only told the night before that it would be postponed because of technical problems. Then days later, we were told our application was rejected.”

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Yeung was last month arrested and charged with allegedly taking part in illegal assemblies during the 2019 social unrest. The case is expected to be heard in July. In one protest in June 2019, he was allegedly hit in the right eye by a police projectile.

His book, A Journey Through the Brick Wall, was among the titles a pro-government group complained about to national security police during last year’s fair. The book is about his eight years as a teacher and his involvement in the 2019 unrest.

Despite the complaints, no action was taken against him or his publishing house.

“One can’t help thinking that political consideration has been involved in TDC’s rejection of our participation. The book fair has always been a thermometer of the freedom of the press and publishing in Hong Kong,” Yeung said.

This year’s Book Fair is expected to be held at the Convention and Exhibition Centre in Wan Chai from July 20 to 26.