Hong Kong's media say about 50 pro-democracy activists were arrested on Wednesday on suspicion of violating the national security law for the territory.
News outlets say the arrests include former members of Hong Kong's Legislative Council and district council members.
They say the details of the charges are unknown, but they are related to pro-democracy primaries held in July to choose candidates for the Legislative Council election.
The Hong Kong government and Beijing warned at the time it might violate the law.
The legislative election was due in September but has been postponed with authorities citing coronavirus risks.
The security law was imposed by Beijing in June. It punishes what China defines as secession, subversion, terrorism and collusion with foreign forces.