Those arrested include Next Digital Chief Executive Cheung Kim-hung and Chief Operating Officer Royston Chow, as well as Ryan Law, Apple's chief editor. They may appear in court as soon as Saturday.
The Apple Daily ran its own story about the raid and said five of its executives were arrested for breaking Article 29 of Beijing's controversial national security law, which prohibits "collusion with a foreign country or with external elements to endanger national security". Apple Daily founder Jimmy Lai is now serving a 20-month prison sentence after being convicted of playing a role in unauthorized protests in 2019, when Hongkongers took the streets in massive antigovernment demonstrations in response to a proposed extradition law that would have allowed suspects to stand trial in China.
Dozens of prominent activists have been arrested since the national security law was introduced previous year.
Prior to the passing of the law, the media in Hong Kong had broadly functioned with freedom that newspapers on the mainland are denied, under the "one country, two systems" model that guaranteed a range of rights following the 1997 handover.
The U.S. and Japan also spoke out about the arrests.
Hong Kong's secretary for security, John Lee, said Thursday's arrests and raid are "not related to normal journalistic work". "Efforts to stifle media freedom and to restrict the free flow of information not only undermine Hong Kong's democratic institutions but they also hurt Hong Kong's credibility and viability as an worldwide hub".
Police cited some 30 articles published by Apple Daily said to have urged foreign governments to impose sanctions on Hong Kong and China.
The paper said computer terminals, hard drives and reporter notepads were among items carted away.
Sharron Fast, a lecturer at the University of Hong Kong's journalism school, described Mr Lee's comments as "ominous and incorrect". Last month, authorities froze Lai's assets and shares in Next Digital. "He urged journalists to "keep a distance from these criminals" though he added", It is your choice whether you will regard them as part of you".
Independent media watchdog the Committee to Protect Journalists has come down heavily on the Chinese authorities for raiding the office of the Apple Daily newspaper in Hong Kong and arresting its five executives including the editor-in-chief.
"They arrived around 7am this morning, our building is besieged", an unnamed reporter said in a live commentary with the broadcast.
Jimmy Lai, the founding chairman of the paper, is serving a 20-month prison sentence after he was convicted in three cases for joining unauthorized assemblies in 2019.
He admitted that the paper was in "crisis" since the jailing of its owner but said his reporters were determined to press on with publishing.
Hong Kong's stock exchange said trading in shares of Next Digital - the publisher of the newspaper - had been halted.
Stand News, a pro-democracy online media outlet, published pictures showing police had smashed down the house door of Chan Pui-man - Law's deputy.