Some 500 policemen raided the offices of pro-democracy paper Apple Daily in Hong Kong, 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 ($2.3m; £1.64m) 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.
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.