Riot police detained a dozen opposition activists as they tried to hold a sit-in in central Moscow.
Policemen rounded up the protesters and bundled them into vans minutes after they unfurled anti-Kremlin banners and sat on the pavement near the downtown Triumfalnaya Ploshchad.
The protesters briefly chanted demands for Prime Minister Vladimir Putin to step down and called for "free elections" at the upcoming parliamentary vote in December.
Wednesday's protests are the latest in a series of actions held on the last day of every month with 31 days. The date is a nod to the 31st article of the Constitution, which guarantees the right of assembly.
Two pro-government youth groups were allowed to stage an anti-opposition rally at nearby Pushkin Square on Wednesday.