Thousands of Russians have been taking part in unauthorised protests to demand the release of the jailed opposition leader Alexei Navalny.
More than 1,000 people have been detained so far, local media say.
In Moscow police have closed metro stations and are restricting movement in the city centre. About 140 people have been detained there.
Mr Navalny was jailed on his return to Russia after recovering from an attempt to kill him with a nerve agent.
The opposition figure had only just arrived from Berlin, where he spent months recovering from the near-fatal attack.
Russian authorities say Mr Navalny was supposed to report to police regularly because of a suspended sentence for embezzlement.
Mr 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.
Mr Navalny has blamed state security agents under Mr 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 Mr Navalny in a video that has gone viral in Russia and has been watched more than 100m times.
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In Moscow the BBC’s Sarah Rainsford says protesters played cat-and-mouse with police, getting up close to officers before retreating to safety. Police snatch squads pulled some protesters through the lines of riot shields. Footage showed a stream of people being escorted on to buses by riot police.
Mr Navalny’s wife Yulia Navalnaya is among those detained, Mr Navalny’s team says. Earlier she posted a picture of herself on the way to the rally.
Police say the protests are illegal, while Russian authorities have warned that the demonstrations could spread the coronavirus.
A 40-year-old protester in Moscow told Reuters she had attended despite having a panic attack the night before over the possible repercussions she could face for taking part.
“I understand that I live in a totally lawless state. In a police state, with no independent courts. In a country ruled by corruption. I would like to live differently,” she said.
In St Petersburg, Mr Putin’s home city, a crowd gathered in a central square and chanted: “Down with the Tsar.”
Rallies in support of Mr Navalny have already taken place in eastern Russia. In the Siberian city of Novosibirsk, at least 2,000 people marched through the city chanting “Freedom” and “Putin is a thief”.
In Yakutsk, where temperatures fell to -40C, a protester named Ivan said it was the first rally he had attended.
“I am tired of the despotism and lawlessness of the authorities. No questions have been answered. I want clarity, openness, and change. This is what made me come here,” he said.
Further rallies saw about 1,000 people demonstrate in Omsk, also in Siberia, and about 7,000 people rally in Yekaterinburg in the Ural region, according to local media reports.
The OVD-Info monitoring group said police had detained more than 1,000 people at protests across the country. The previous week’s protests saw 4,000 people detained.
A number of close associates of Mr Navalny have been detained since last week and others, including his brother and Pussy Riot activist Maria Alyokhina, have been put under house arrest.
The chief editor of a Russian website specialising in human rights, Sergei Smirnov, was also arrested outside his home on Saturday. News of his detention, apparently over allegations he participated in last week’s protests, has been condemned by other journalists.
In Moscow, police have reportedly been struggling to find space in jail for supporters of the opposition leader.