Police in Hong Kong have arrested a pro-democracy newspaper owner under a new national security law and police raided his offices in a deepening crackdown on dissent in the city.

Jimmy Lai was among seven people arrested in an operation focused on his Next Media publishing group, the latest to target dissidents since China imposed the sweeping law on Hong Kong at the end of June.

In a statement, police said the seven were detained on suspicion of colluding with foreign forces - one of the new national security offences - and fraud.

Journalists working at Mr Lai's Apple Daily newspaper took to Facebook to broadcast images of police officers conducting the raid.

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 Mr Lai, 72, was present, in handcuffs and surrounded by officers.

In a statement police said the search was conducted with a court warrant, which they said was shown to staff.

Jimmy Lai was arrested under new security laws 

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.

Mr Lai's Apple Daily and Next Magazine are pro-democracy and critical of China.

Few Hong Kongers generate the level of personal vitriol from the Chinese government that Mr Lai does.

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For many residents of the city he is an unlikely hero - and the only tycoon willing to criticise Beijing.

But in China's state media he is a "traitor", the biggest "black hand" behind last year's protests and the head of a new "Gang of Four" conspiring with foreign nations to undermine the motherland.

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

Mr Lai said in mid-June, two weeks before the new security law was imposed on Hong Kong: "I'm prepared for prison..If it comes, I will have the opportunity to read books I haven't read. The only thing I can do is to be positive."

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

Police cordon off the area outside the Next Media publishing offices which are owned by Jimmy Lai

In the June interview with AFP, Mr Lai described Beijing's new security law as "a death knell for Hong Kong" and said he feared authorities would come after his journalists.

The 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 criminalise certain political speech overnight, such as advocating for sanctions, greater autonomy or independence for Hong Kong.

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

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 Carrie Lam postponed the elections for a year, citing a surge in coronavirus cases.