HONG KONG — In the highest-profile attack yet on free speech and press freedom in Hong Kong, police on Monday arrested Jimmy Lai, a prominent pro-democracy media tycoon, and raided the offices of his newspaper, demonstrating China’s resolve to silence dissent and bring the city to heel.
The arrest was the most striking in a series of moves against critics and democracy advocates by Beijing and the allied administration of Hong Kong, in the six weeks since China adopted a sweeping new national security law. And it validated fears that the authorities would make aggressive use of the law, aiming to smother the territory’s freewheeling press and political culture.
“It just gives the lie to any assurances that the national security law would just target a few people involved in rioting, said Keith Richburg, director of the University of Hong Kong’s journalism school. “It’s put a chilling effect over everything here.”
Mr. Lai and his media company, long a thorn in the side of the Chinese Communist Party, supported the antigovernment, pro-democracy protests that gripped the city last year. On Monday, police officers led him out of his mansion in handcuffs and, hours later, more than 200 officers filed into the newsroom of his newspaper, Apple Daily, and rifled through desks.
Some Apple Daily reporters livestreamed video of the raid, documenting a story unfolding in their own offices, while others looked on in stunned silence. When one asked Mr. Lai about the arrest and the raid, he replied gruffly, “How should I think about it, dude?”
The police also arrested Mr. Lai’s two sons, who are not involved in his media business, and four executives from his company, Next Digital, including its chief executive, Cheung Kim-hung.
Since the security law took effect, the Hong Kong authorities have arrested people for T-shirts and tweets that were seen as advocating independence from China, have asserted a right to prosecute critics abroad, have barred 12 pro-democracy candidates from legislative elections and have postponed those elections by a year.
Those moves, like the arrest of Mr. Lai, signaled that China means to deal with Hong Kong as it pleases, unconstrained by international disapproval.
Australia, Canada, New Zealand, Britain and the United States said in a joint statement on Sunday that they were “gravely concerned” by the disqualification of candidates and the security law.
On Friday, the Trump administration placed sanctions on Carrie Lam, Hong Kong’s chief executive, and 10 other senior officials over their roles in suppressing dissent. China retaliated on Monday by sanctioning 11 American nonprofit leaders and lawmakers, including Senators Marco Rubio and Ted Cruz.
Mr. Lai, 72, was born in the mainland city of Guangzhou and worked his way up through Hong Kong’s textile industry to build the Giordano clothing brand. He embraced democracy and publishing after the 1989 crackdown on pro-democracy protests around Tiananmen Square in Beijing, but had to sell his clothing business when the Chinese authorities retaliated against its mainland outlets.
Mr. Lai is often denounced by Chinese officials and pro-Beijing news outlets, who have called him one of the leading forces behind the large antigovernment protests in Hong Kong.
Apple Daily is a fiercely pro-democracy newspaper that regularly criticizes the Hong Kong government and the Chinese leadership. It gives extensive coverage in print and online to the protest movement, and runs front page headlines reminding readers of when to take to the streets.
The newspaper reported Monday that the police had accused Mr. Lai of collusion with a foreign country or external elements, a vaguely defined crime under the national security law.
Mark Simon, a senior executive with Next Digital, said that Mr. Lai’s two sons, in addition to facing national security charges, were also being investigated for unspecified violations of corporate law. Given the lack of involvement by Mr. Lai’s sons in Next Digital, the police actions suggested that the authorities were investigating Mr. Lai’s private investments.
The police said in a statement on Facebook that officers had entered a building in Tseung Kwan O, the location of Apple Daily’s headquarters, with a search warrant in order to investigate national security offenses. An Apple Daily reporter who was narrating a livestream video said that police officers had loaded bags of documents taken from the building onto a truck.
The live footage showed a tense scene in the newsroom. When an editor demanded to know the exact boundaries of the area being searched, he was shoved by shouting officers. “Remember his face,” an inspector said, raising his index finger. “If he still behaves like this, give him a warning. And if he doesn’t listen to the warning, arrest him.”
Livestream footage also showed plainclothes officers at a restaurant owned by one of Mr. Lai’s sons in Hong Kong’s Central district. The officers loaded a crate filled with electronic devices they had seized into a private vehicle and did not respond when reporters asked if they were national security officers and whether they had search warrants.
By Monday afternoon, the police said they had arrested two more people, for a total of nine, aged 23 to 72, on suspected violations of the security law. Wilson Li, a freelance journalist who works for ITV News, was one of those arrested, the broadcaster said in a statement. However his arrest appeared to be related to activist organizing, not journalism. Another activist, Andy Li, was also arrested, ITV reported.
In the evening, the police arrested Agnes Chow, a prominent activist and politician, on suspected national security law violations of inciting secession, her lawyer said.
“I’m a bit scared,” Ms. Chow wrote in a Facebook post on Sunday that noted strange men had been standing outside her apartment in shifts. “But I believe in what I’m doing.”
In recent years, media outlets in Hong Kong have complained about the erosion of press freedom in the city. Journalists covering protests have been pepper-sprayed and detained by the police. Pro-Beijing lawmakers have called for reporters to be registered. And outlets like Apple Daily have faced advertising boycotts from companies that fear retaliation from mainland authorities.
Foreign journalists, including some working for The New York Times, have experienced unexplained delays in renewing visas, and the Hong Kong authorities have refused to renew the work visa of one Times correspondent. The Times said last month that it would move part of its Hong Kong operations to Seoul, South Korea, in response to uncertainty created by the security law.
Still, the sight of media executives in handcuffs and the police raiding a newsroom elicited a new level of fear.
“Those scenes were kind of shocking to wake up to,” said Mr. Richburg, the journalism school director. “It’s hard to believe this is Hong Kong. It is incredible how quickly everything has changed.”
The Foreign Correspondents’ Club of Hong Kong said in a statement that the raid and arrests were “a direct assault on Hong Kong’s press freedom and signal a dark new phase in the erosion of the city’s global reputation.” It added, “Today’s events raise worries that such actions are being used to erase basic freedoms in Hong Kong.”
Mr. Lai was previously arrested in February and again in April over accusations that he had participated in unauthorized protests last year. He also faces charges for joining an unauthorized vigil on June 4 to mark the anniversary of the 1989 Tiananmen crackdown by Beijing.
It was not immediately clear what action by Mr. Lai that the authorities were considering to be possible collusion with a foreign power.
Mr. Lai, who also has British citizenship, traveled to Washington last year and met with Vice President Mike Pence, Secretary of State Mike Pompeo and the speaker of the House, Nancy Pelosi, among others. The national security law stipulates, however, that it can be applied only to activities that occur after it went into force, at the end of June.
Mr. Lai previously said he believed that the new law would be used against him. Soon after he first wrote about the legislation, the Communist Party-owned Global Times newspaper cited mainland experts who said his tweets had provided “evidence of subversion.”
“I have always thought I might one day be sent to jail for my publications or for my calls for democracy in Hong Kong,” he wrote in an Op-Ed article for The Times. “But for a few tweets, and because they are said to threaten the national security of mighty China? That’s a new one, even for me.”