Some 500 policemen raided the offices of pro-democracy paper Apple Daily, alleging its reports breached a national security law.
Police also arrested the editor-in-chief and four other executives at their homes. It also froze HK$18m ($2.3; £1.64) of assets owned by three companies linked to Apple Daily.
The paper is owned by Jimmy Lai, who is in jail on a string of charges.
Apple Daily is known to be critical of the mainland Chinese leadership.
In a press briefing, police said that since 2019, Apple Daily had published more than 30 articles calling on countries to impose sanctions on Hong Kong and mainland China.
It added that the assets of Apple Daily Limited, Apple Daily Printing Limited and AD internet Limited had been frozen.
Dozens of prominent activists have been arrested since the national security law was introduced last year.
Police entered the Apple Daily office premises at around 07:30 local time on Thursday (23:30 GMT Wednesday) and blocked off all entrances and exits,
The paper broadcasted live footage of the raid on its Facebook account.
Police said in a statement that they conducted a search operation at a media company, adding that their warrant “covered the power of searching and seizure of journalistic materials”.
Photos published online by Apple Daily showed police going through reporters’ computers.
The police separately visited the homes of editor-in-chief Ryan Law, parent company Next Digital’s CEO Cheung Kim-hung, COO Chow Tat-kuen, Apple Daily publisher Chan Pui-man and Director Cheung Chi-wai, and arrested them.
Police did not name those arrested but confirmed that the five people, aged between 47 and 63, were arrested for “collusion with a foreign country or with external elements to endanger national security”.
This is the second raid to take place in Apple Daily in under a year – a raid last August saw 10 people arrested including Lai and his sons.
How significant is this?
According to the BBC’s Danny Vincent, this incident marks the first time Hong Kong journalists have been arrested under suspicion of violating the national security law.
In a press conference on Thursday, Hong Kong security chief John Lee said Apple Daily’s actions “targeted the use of journalistic work as a tool to endanger national security”.
Mr Lee also added that they were not “normal journalists”, calling on others to “keep a distance from them”.
Some reporters from the paper told the BBC they planned to continue printing, but they now fear that today’s events – both the arrests and the freezing of assets – may mark the beginning of the end for Apple Daily.
Who is Jimmy Lai?
Lai is one of the most prominent supporters of Hong Kong’s pro-democracy movement.
Estimated to be worth more than $1bn (£766m), he made his initial fortune in the clothing industry and later ventured into media and founded Next Digital.
In his last interview with the BBC before he was sentenced to jail, he said he would not give in to intimidation.
“If they can induce fear in you, that’s the cheapest way to control you and the most effective way and they know it. The only way to defeat the way of intimidation is to face up to fear and don’t let it frighten you,” he said.
He is currently facing time in jail for a series of charges, including participating in an unauthorised assembly in 2019.
What is the national security law?
The law was introduced in 2020 in response to massive pro-democracy protests that swept the city state the previous year.
It essentially reduced Hong Kong’s judicial autonomy and made it easier to punish demonstrators. It criminalises secession, subversion and collusion with foreign forces with the maximum sentence life in prison.
Beijing said the law would target “sedition” and bring stability, but critics have said it violates the agreement under which Britain handed back Hong Kong to China in 1997.
The handover agreement saw the creation of the Basic Law under the “one country, two systems” principle.
This is supposed to protect certain freedoms for Hong Kong: freedom of assembly and speech, an independent judiciary and some democratic rights – freedoms that no other part of mainland China has.
Fears that this model was being eroded were what led to the huge pro-democracy protests in 2019.
Since the law was enacted in June, more than 100 people have been arrested under its provisions, including Lai.