Several hundred people from leftist movements gathered in central Moscow on Saturday to protest against officials' arbitrariness and defend their rights in other spheres.
The rally, entitled the Day of People's Wrath, was sanctioned by authorities.
"Suddenly about 50 representatives of the Molodaya Gvardiya [Young Guard] movement tried to join the rally and started throwing leaflets with various slogans. Policemen stopped their activity and detained 43 people," a police spokesman said.
The detainees face fines.