Hong Kong pro-democracy media tycoon Jimmy Lai was arrested Monday and led in handcuffs through his newspaper office as police raided the building, part of a sweeping crackdown on dissent since China imposed a security law on the city.

Lai, 71, was among seven people detained on suspicion of colluding with foreign forces -- one of the new national security offences -- and fraud in an operation targeting the Next Media publishing group. 

It was the latest police operation against dissidents since Beijing imposed the law on Hong Kong at the end of June. Two of Lai's sons were among those detained, a police source told AFP.

Journalists working at Lai's Apple Daily took to Facebook to broadcast dramatic footage of police officers conducting the raid, and the newspaper's chief editor Law Wai-kwong demanding a warrant from officers.

"Tell your colleagues to keep their hands off until our lawyers check the warrant," he was filmed saying.

Apple's staff were ordered to leave their seats and line up so police could check their identities as officers conducted searches across the newsroom.

At one point Lai was present, in handcuffs and surrounded by officers. 

Police said the search was conducted with a court warrant which was shown to staff.

Chris Yeung, president of the Hong Kong Journalists Association, described the police action as "shocking and terrifying". 

"This is unprecedented, and would be unimaginable only one or two months ago," he said.

Apple editor Law later sent a note to staff, seen by AFP, telling reporters to "stand by their posts" as he vowed to get the latest edition printed despite the raid.

'Prepared for prison'

The security law was introduced in a bid to quell last year's huge and often violent pro-democracy protests, and authorities have since wielded their new powers to pursue the city's democracy camp, sparking criticism from western nations and sanctions from the United States. 

Lai's Apple Daily and Next Magazine are unapologetically pro-democracy and critical of Beijing. 

Few Hong Kongers generate the level of personal vitriol from Beijing that Lai does.

China routinely calls him a a "traitor" and a "black hand" behind last year's protests.

Allegations of Lai colluding with foreigners went into overdrive in state media last year when he met with US Secretary of State Mike Pompeo and Vice President Mike Pence.

Lai spoke to AFP in mid-June, two weeks before the new security law was imposed on Hong Kong.

"I'm prepared for prison," he said.

He described Beijing's new security law as "a death knell for Hong Kong" and said he feared authorities would come after his journalists. 

He also brushed off the collusion allegations, saying Hong Kongers had a right to meet with foreign politicians.

Sweeping new law

Beijing's new law targets secession, subversion, terrorism and colluding with foreign forces.

Both China and Hong Kong have said it will not affect freedoms and only targets a minority.

But its broadly worded provisions criminalised certain political speech overnight, such as advocating sanctions, greater autonomy or independence for Hong Kong.

Critics, including many Western nations, believe the law has ended the key liberties and autonomy that Beijing promised Hong Kong could keep after its 1997 handover by Britain. 

Washington last week responded by imposing sanctions on a group of Chinese and Hong Kong officials -- including the city's leader Carrie Lam.

The law's introduction has coincided with ramped up police action against democracy supporters.

About two dozen -- including Lai -- have been charged for defying a police ban to attend a Tiananmen remembrance vigil in early June. Lai and many others are also being prosecuted for taking part in last year's protests.

Last month a dozen high-profile pro-democracy figures were disqualified from standing in local elections for holding unacceptable political views.

The banned opinions included being critical of the security law and campaigning to win a majority in the city's partially-elected legislature in order to block government laws.

Shortly after the disqualifications, city leader Lam postponed the elections for a year, citing a surge in coronavirus cases.