Moscow police detained 49 people at an unauthorized opposition protest on Triumfalnaya Square on Tuesday, a police spokesman said.
"Administrative penalties were imposed on the detainees and they face administrative sanctions for violating rules for public events,” the source said.
About 100 people took part in the rally, which was also attended by about 40 journalists.
A RIA Novosti correspondent reported that the protest began at 6:00 p.m. The venue is almost empty at the moment but police cordons remain in place.
Triumfalnaya is the regular venue for unsanctioned Strategy-31 protests held on the 31st day of a month to affirm Article 31 of the Constitution, which guarantees freedom of assembly.