Hong Kong police declared the Apple Daily newspaper office a crime scene Thursday, after 500 officers descended on the premises to arrest executives and top editors and seize journalistic materials under the city's national security law.

Apple Daily said Thursday that the company's CEO Cheung Kim Hung, COO Chow Tat Kuen and chief editor Ryan Law, along with the deputy chief editor and online editor were all arrested and accused of colluding with foreign forces to endanger national security — a provision of the sweeping legislation introduced last year that banned sedition, secession and subversion against Beijing.

"This case involves a conspiracy," Hong Kong Security Secretary John Lee told reporters at a press conference Thursday, adding that the police raid was targeted at those who use journalism as a "tool to endanger national security."

Senior superintendent Steve Li, of the police's national security department, said 18 million Hong Kong dollars ($2.3 million) in assets related to the newspaper had been frozen. The assets are owned by three companies: Apple Daily, Apple Publishing & A.D. Internet Limited.

The publication live-streamed the early morning raid on its Facebook page, showing police asking staff to show proof of identity, and blocking them from returning to their desks.

The Hong Kong government confirmed Thursday that it had arrested five "directors of a company" on suspicion of violating the national security law and that officers had obtained a search warrant which gave officers the power to seize journalist material.

After the initial raid, Li said Apple Daily's headquarters was now a crime scene, and that officers had confiscated electronic devices, such as mobile phones, computers and laptops. In images published online by Apple Daily, police officers could be seen examining computers at the office.

He said police were investigating Apple Daily for its earlier attempts to "colluding with foreign forces and external elements to endanger national security." Li said that since 2019 Apple Daily had published articles calling on foreign countries to sanction the Chinese and Hong Kong governments.

Last year, the deputy director of the Hong Kong and Macao Affairs Office of the State Council said the national security law, which came into effect on June 30, would not be retroactive.

Li also urged the public not to repost the content. "If you have no real reason to share these types of articles, I would advise everyone not to," Li warned.

"The government is raiding Apple Daily because they can't shut it down by economic pressure," Mark Simon, managing director of the private holdings of newspaper's controversial founder Jimmy Lai, told CNN Business. "So that means we have 130 cops in our newsroom vacuuming up documents."

The arrests and probe are the latest step in an escalating crackdown against the provocative, anti-Beijing tabloid, which has become the poster child in Hong Kong for media freedom in what many analysts argue is an increasingly hostile landscape for the industry.

Media mogul Lai — who for decades has been a symbol of the city's tensions with mainland China — already faces charges under the national security law and is currently serving jail sentences for his role in unauthorized assemblies dating from the 2019 pro-democracy protests.

Li said that Thursday's operation was not targeting the press, but one individual organization that violated the national security law, saying the Hong Kong government values press freedom.

Carrie Lam, the city's leader, said after the passage of the Beijing-imposed national security law that Hong Kong people should still be able to enjoy freedom of speech and press. The government has also told CNN Business in the past that it is "firmly committed to protecting and respecting the freedom of the press."

Lee, the security secretary, sought to quell concerns about press freedoms in the city on Thursday, saying that the probe was targeted at actions that were "not normal journalistic work."

"Please understand that our actions are not targeting journalistic work," he added. 'We target perpetrators who use journalistic work as a tool to endanger acts of national security."

Press freedoms are a fundamental right guaranteed by Hong Kong's Basic Law, the city's mini-constitution. But experts have worried about the usage of the national security law, and there have been indications of pressures on other media in the city. Earlier this year, a freelance producer working with public broadcaster Radio Television Hong Kong (RTHK) was convicted and fined for violating a traffic ordinance — a convoluted charge connected to her reporting on a pivotal event during the height of the 2019 protests.

A recent ranking of worldwide press freedoms, meanwhile, indicates that the environment in Hong Kong has deteriorated. The international watchdog Reporters Without Borders — which qualifies such freedoms based on data on abuse and acts of violence against journalists along with a questionnaire to experts — ranked Hong Kong 80 out of 180 countries for press freedom, down from 18 out of 138 in 2002.

- by Eric Cheung, CNN Business