'Dark new phase' in Hong Kong as pro-democracy media tycoon Jimmy Lai is arrested, his newsroom raided

Once seen as the bastion of free press and speech in Asia, Hong Kong now appears to be changing day by day.

Police detained the city's most prominent media tycoon and Beijing critic, Jimmy Lai, under the new national security law on Monday morning.

Hundreds of police officers also stormed the newsroom of his pro-democracy media outlet, Apple Daily, rummaging through documents on journalists' desks as Lai was led through the office with handcuffs.

Lai's arrest, for suspected collusion with foreign forces, is the highest-profile use of the sweeping new legislation, imposed by Beijing on June 30th following last year’s protests.

It is not clear what specific action led to the 72-year-old’s arrest.

The law punishes anything Beijing deems subversion, secession, terrorism and collusion with foreign forces with sentences of up to life in jail.

Apple Daily said Lai's son was detained along with executives at the paper.

Hong Kong police confirmed that at least nine people, aged 23 to 72, were arrested on suspicion of breaching the new law.

The raid was live streamed on Apple Daily’s official Facebook page.

A journalist who works at Apple Daily spoke to ABC News on condition of anonymity.

He was expected to start work later in the day but received a message from his supervisor in the morning not to come into the office.

“I was prepared for this day to come but I didn’t expect it to come so quick and in the way that it did. The police were gravely intruding on journalists’ privacy and the confidentiality of their news materials,” said the local Hong Kong resident.

He describes feeling “sad, angry and scared” over the ordeal.

Pro-democracy and press advocacy groups were quick to condemn Monday’s events, with the Hong Kong Journalist Association warning journalists in the city to prepare for further crack downs on reporters.

Hong Kong’s Foreign Correspondents’ Club said it signaled “a dark new phase in the erosion of the city’s global reputation.”

Amnesty International called it “a disturbing demonstration of how the Hong Kong authorities intend to use the new national security law to threaten press freedom.”

The security law has been widely condemned by Western governments which accuse Beijing of breaking its promise of allowing the territory semi-autonomy under the "One Country, Two Systems" framework.

Monday’s newsroom raid comes as reports emerge of a new unit within the Hong Kong Immigration Department that is now vetting foreign journalist work visas.

Later in the day it emerged that local freelance journalist, Wilson Li, was among those arrested.

Li, whose clients include UK’s ITN, used to be a member of student activist group Scholarism.

Washington and Beijing have been engaged in a tit-for-tat spat over journalists as part of a wider cold war between the world’s two largest economies.

Also on Monday, Beijing rolled out expected retaliatory sanctions on 11 U.S. officials, including Republican senators Marco Rubio and Ted Cruz.

Last week the U.S. announced sanctions on Chief Executive Carrie Lam and 10 other Hong Kong and Beijing officials.

Hu Xijin, editor-in-chief of the Chinese state-run Global Times, said the arrest of Lai proves that the Hong Kong authorities won’t be intimidated by the U.S. sanctions.

Hu tweeted that Washington’s punitive measures “are pushing HK civil servants further to Beijing…In the future, the sanctions will also push the hearts and minds of entire HK society to the Chinese mainland, promoting China’s unity."

The timing of today’s raid and new round of sanctions is also raising eyebrows.

On Sunday U.S. Health Secretary Alex Azar began his visit to Taiwan, something likely to irk Beijing, which considers the autonomous island a renegade province.

Veteran democrat Albert Ho told ABC News he believes “the unprecedented scale” of Monday's police raid is “a retaliation, to show to the U.S. government about the anger arising from the imposition of sanctions.”

Ho was charged along with Lai and more than 20 others last Friday for attending an event to mark the 1989 Tiananmen Square crackdown on June 4.

Lai and Ho are part of what Chinese state-media have dubbed the "gang of four", accusing them of instigating unrest, along with Democratic Party founder Martin Lee and former chief secretary Anson Chan.

Ho said that he has long conceived the possibility that “this long arm may extend to me.”

Ho, who runs a law firm in the city, said, “I won’t allow myself to be scared and intimidated. I will stand firm that I have done nothing wrong against my country. I’ve only advocated for my country to democratize and that one party dictatorship should be reformed into a democratic system.”