Russian police have detained more than 2,000 people during nationwide protests in support of jailed opposition leader Alexei Navalny, monitors say.
Tens of thousands of people defied a heavy police presence to join the rallies, a huge show of defiance against President Vladimir Putin, according to the BBC.
In Moscow, riot police were seen beating and dragging away protesters.
Mr. Navalny, President Putin's most high-profile critic, called for protests after his arrest last weekend.
He was detained on January 17 after he flew back to Moscow from Berlin, where he had been recovering from a near-fatal nerve agent attack in Russia last August.
On his return, he was immediately taken into custody and found guilty of violating parole conditions. He says it is a trumped-up case designed to silence him, and has called on his supporters to protest.
Prior to the rallies, Russian authorities had promised a tough crackdown, with police saying any unauthorised demonstrations and provocations would be "immediately suppressed." Several of Mr. Navalny's close aides, including his spokeswoman Kira Yarmysh, were arrested earlier in the week.
Read alsoNavalny supporters being beaten, arrested as protests begin to demand his release – mediaOVD Info, an independent NGO that monitors rallies, said more than 2,100 people had been detained during protests, more than 700 of them in Moscow alone.
Mr. Navalny's wife, Yulia, said she had been detained at a protest and later released.
Teenagers were among the many Navalny supporters who joined the demonstration in Moscow's central Pushkin Square. They were later forced by police to disperse to neighbouring streets.
Russia's interior ministry said 4,000 had turned up in Moscow, but opposition sources and reporters on the ground say it was in the tens of thousands.
Among them was Lyubov Sobol, a prominent aide of Mr. Navalny who had already been fined for urging Russians to join the protests. She tweeted a video of police roughly pulling her away from an interview with reporters.
Mr. Navalny's wife, Yulia, also said she was being held by police at the same protest, posting an image on her Instagram account with the caption: "Apologies for the poor quality. Very bad light in the police van."
Prominent Navalny activists are also being held in the cities of Vladivostok, Novosibirsk and Krasnodar.