Hong Kong's media say about 50 pro-democracy activists were arrested on Wednesday morning on suspicion of violating the national security law for the territory.
News outlets say the arrests include Wu Chi Wai, former Legislative Council member and former head of the Democratic Party, and Jimmy Sham, a district council member who formerly headed a pro-democracy group that organized massive protests.
Details of the charges are still unknown. But the media say the allegations are related to the primaries held in July to narrow down pro-democracy candidates for the Legislative Council election.
Winning a majority in the city legislature would have enabled the rejection of the government's budget bill and forced Hong Kong Chief Executive Carrie Lam to resign.
Beijing said the attempt could be seen as an act of subversion in violation of the national security law.
The legislative election was due in September but has been postponed with authorities citing coronavirus risks.
The new security law was imposed by Beijing in June. It punishes what China defines as secession, subversion, terrorism and collusion with foreign forces.
The arrests are regarded as the first application of the law over a suspected act of subversion.