The lead editorial writer of Hong Kong pro-democracy newspaper Apple Daily was arrested under a powerful new national security law on Wednesday, days after police raided its newsroom, arrested five executives and froze its assets.

Police said a 55-year-old man was arrested "on suspicion of colluding with foreign forces to endanger national security".

A police source told AFP the arrested man wrote columns for Apple Daily under the pen name Li Ping.

Apple Daily said Li Peng, whose surname is Yeung, was one of the paper's top columnists and the lead writer of their editorials -- which express the editorial board's official views.

Records show Li Ping has written nearly 800 commentaries over the past five years. The last one published on Tuesday touched on academic freedoms in mainland China.

Hong Kong police have raided Apple Daily's newsroom and arrested executives and a columnist at the outspoken tabloid
Hong Kong police have raided Apple Daily's newsroom and arrested executives and a columnist at the outspoken tabloid AFP / Peter PARKS

The arrest widens the police operation against Apple Daily, which is facing the threat of imminent closure.

The outspoken tabloid has long been a thorn in Beijing's side, with unapologetic support for the city's pro-democracy movement and caustic criticism of China's authoritarian leaders.

Its owner Jimmy Lai is in jail and was among the first to be charged under the security law after Beijing imposed it on the city last year following 2019's huge democracy protests.

Last Thursday, more than 500 police officers raided the paper's newsroom over what authorities said were articles and columns appealing for sanctions against China.

Five executives, including chief editor Ryan Law and CEO Cheung Kim-hung, were arrested under the charge of colluding with foreign forces.

Apple Daily is facing imminent closure
Apple Daily is facing imminent closure AFP / ISAAC LAWRENCE

Law and Cheung were officially charged on Saturday and remanded into custody.

Authorities also froze HK$1.8 million (US$2.3m) in assets owned by Apple Daily using the security law, crippling the paper's ability to continue operating.

The security law, written in Beijing and imposed on Hong Kong last June, allows authorities to freeze assets of any individual or company in the international business hub that is deemed a security threat.

It does not require a court order.

Board members of the paper's parent group Next Digital have asked Hong Kong's Security Bureau to unfreeze some of its assets so it can pay staff.

If that does not happen, the paper said it will cease publishing with a final print date to be announced on Friday.

In a Facebook update, the paper's union said it had been told Saturday would be the last edition if management decide to fold.

The paper's financial news desk, its English edition and the media group's weekly magazine have all since announced that they have ceased publication.

Authorities have not released a list of Apple Daily's articles or columns that have been deemed a national security crime.

In an editorial on Friday, Li Ping wrote that Hong Kong authorities were drawing "layers of red lines" around journalism and said the future of press freedom in the city was now "precarious".