At least 260 people have been arrested while protesting against Russian President Vladimir Putin in Moscow, police say.
The protesters are believed to be linked to nationalist politician Vyacheslav Maltsev's Artillery Preparation movement.
It has been declared extremist and is banned in Russia.
Mr Maltsev says a revolution is imminent. Last year in a debate he called for Mr Putin to be impeached.
He is reported to be living abroad after a Moscow court issued an arrest warrant for him over his calls for extremist measures to overthrow the government.
The Tass news agency quoted police and independent monitors as saying supporters of the radical opposition group convened beside the Kremlin in central Moscow.
Most of the arrests were made in the Russian capital but people were also detained in five other cities.
Some of the protesters in Moscow were reported to be carrying knives, knuckledusters and rubber bullet-firing pistols.
An AFP photographer said police wearing helmets and bullet-proof vests at one point confronted the protesters in central Moscow.
Many of those arrested were reported to be men in their twenties and some are believed to have been released without charge soon afterwards.
Mr Maltsev ran for a parliament last year and runs a popular YouTube channel which is frequently critical of the government.
The protest by his supporters on Sunday follows on from another demonstration in Moscow on Saturday in which police detained scores of nationalist anti-Kremlin activists.