Police seized Ales Chyhir, a city councilor and a history teacher, shortly after he chained himself to a lamppost near the building of Babruysk city council, in the eastern region of Mahileu.
Twelve other people, including 11 reporters who were covering Chyhir's protest action, were also taken to a police station. An RFE/RL Belarus Service correspondent was among them.
The reporters, including the RFE/RL Belarus Service correspondent, were released after several hours. Chyhir and another youth opposition activist remain in custody.
RFE/RL's reporter Halina Abakunchyk said the police erased some of the pictures and recordings the reporters had made.
Chyhir was the head of the local campaign headquarters of Alyaksandr Milinkevich, the united democratic opposition challenger of President Alyaksandr Lukashenka in this year's presidential election.