Hong Kong pro-democracy media mogul Jimmy Lai on Monday was arrested on suspicion of breaching the national security law for the Hong Kong Special Administrative Region and police raided his newspaper offices.

Lai was among 10 arrested in an operation focused on his Next Media publishing group, the latest to target dissidents since Beijing imposed the law on Hong Kong at the end of June.

Among the others arrested were two of Lai’s sons, young pro-democracy activist Agnes Chow and Wilson Li, a former activist who describes himself as a freelance journalist working for Britain’s ITV News.

Mark Simon, a close aide of Lai’s said: “They arrested him at his house at about 7am.”

In a statement, police said seven people, aged from 39 to 72, were detained on suspicion of colluding with foreign forces – one of the new national security offences – and fraud.

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

In the footage, the newspaper’s editor-in-chief Ryan Law Wai-kwong can be seen demanding a warrant from officers.

Law was filmed saying: “Tell your colleagues to keep their hands off until our lawyers check the warrant.”

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 72-year-old Lai 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.

The security law was introduced in a bid to quell last year’s huge and often violent pro-democracy protests and has sparked criticism from western nations and sanctions from the US.

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.

For many residents of the city, he is an unlikely hero – a pugnacious, self-made tabloid owner and the only tycoon willing to criticise Beijing.

But in China’s 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 Lai colluding with foreigners went into overdrive last year when he met with US Secretary of State Mike Pompeo and Vice-President Mike Pence.

Two weeks before the new security law was imposed on Hong Kong, Lai said: “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.