Thousands of Russians have been taking part in unauthorised protests to demand the release of the jailed opposition leader Alexei Navalny.
More than 5,000 people have been detained, a monitoring group says. In Moscow police closed metro stations and blocked off the city centre.
Navalny was jailed on his return to Russia after recovering from an attempt to kill him with a nerve agent.
He blames the security services for the attack but the Kremlin denies this.
The opposition figure was arrested after arriving in Moscow from Germany, where he spent months recovering from the near-fatal incident.
Russian authorities say Navalny was supposed to report to police regularly because of a suspended sentence for embezzlement.
Navalny has denounced his detention as “blatantly illegal”, saying the authorities had allowed him to travel to Berlin for treatment for the Novichok poisoning, which happened in Russia last August.
Navalny has blamed state security agents under Putin’s orders for the attempt on his life and investigative journalists have named Russian FSB agents suspected of the poisoning. But the Kremlin denies involvement and disputes the conclusion, by Western weapons experts, that Novichok was used.
Meanwhile, Russian President Vladimir Putin has denied reports he is the owner of a vast palace on the Black Sea, as alleged by Navalny in a video that has gone viral in Russia and has been watched more than 100m times.