HONG KONG (AP) — An aide to Hong Kong media tycoon Jimmy Lai says Lai was arrested Monday morning under the city’s national security law on suspicion of collusion with foreign powers.

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

Hong Kong police said in a statement that seven people had been arrested on suspicion of violating the national security law, but did not reveal the names of those arrested.

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

Simon said that police searched both Lai and his son’s home, as well as other members of media group Next Media, which Lai founded.