Eduard Limonov, leader of the unregistered Other Russia party, was released along with some 30 other participants in a protest rally at Moscow’s Triumfalnaya Square, a city police spokesman said on Saturday.
The protesters will have to appear before a court later.
Police said about 30 people were detained on Friday “for attempting to hold an unsanctioned rally.”
Another 14 people were detained in St. Petersburg for attempting to hold a parallel unsanctioned rally there.
In June, Russia passed a bill hiking maximal fines for unsanctioned rallies from 1,000 rubles to 300,000 rubles ($30 to $9,300).
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.