Hong Kong police arrested media tycoon Jimmy Lai and raided the publisher's headquarters Monday in the highest-profile use yet of the new national security law Beijing imposed on the city after protests last year.

“Jimmy Lai is being arrested for collusion with foreign powers at this time,” Mark Simon wrote on Twitter.

Hong Kong police said seven people between 39 and 72 years old had been arrested on suspicion of violating the new security law, but the statement did not reveal the names of those arrested.

Lai, 71, owns popular tabloid Apple Daily and is an outspoken pro-democracy figure in Hong Kong who regularly criticizes China’s authoritarian rule.

The security law came into effect June 30 and is widely seen as a means to curb dissent after anti-government protests rocked Hong Kong last year.

Simon said that police searched both Lai and his son’s home, and detained several other members of media group Next Digital, which Lai founded.

Over a hundred police also raided Next Digital's headquarters in Hong Kong, entering the newsroom and searching the desks. Simon said in a tweet that the police were executing a search warrant.

Next Digital operates the Apple Daily tabloid, which Lai founded in 1995, ahead of Britain’s handover of Hong Kong to China. Like Lai, Apple Daily has a strong pro-democracy stance and often urged its readers to take part in pro-democracy protests.

The security law outlaws secessionist, subversive and terrorist acts, as well as collusion with foreign forces in the city's internal affairs. The maximum punishment for serious offenders is life imprisonment.

Last month, Chinese broadcaster CCTV said pro-democracy activist Nathan Law and five others were wanted under the law, although all six had fled overseas. Law had relocated to Britain in July to continue international advocacy work for Hong Kong.