Hong Kong media tycoon Jimmy Lai was arrested on Monday on suspicion of collusion with foreign powers, his aide said, 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 had been arrested on suspicion of violating the national security law, but the statement 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.
The national 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.