A total of 120 people were detained during an unsanctioned rally in support of freedom of assembly in downtown Moscow on Thursday, a police spokesman said.
Most of the detained protesters have been released by Friday morning, police later reported.
Russian opposition groups hold rallies in Moscow, St. Petersburg and other cities on the last day of each month that has 31 days in defense of their right to freedom of assembly, as enshrined in Article 31 of the Russian Constitution.
Police have estimated the number of protesters who gathered on Thursday on Moscow’s Triumfalnaya Square at 150, adding that some 200 journalists were covering the protest, which also attracted nationalist supporters.
The opposition Novaya Gazeta newspaper said some 300 people took part in the rally.
Eduard Limonov, the leader of the Other Russia unregistered party, was among those detained.
Another eight protesters were detained during a similar rally in St. Petersburg.