Russia's main opposition leader, Alexei Navalny, has been arrested at an anti-corruption protest he organised in the capital, Moscow.
Thousands of people have joined rallies nationwide, calling for the resignation of Prime Minister Dmitry Medvedev over corruption allegations.
Most of the marches were illegal, organised without official permission.
Police have been deployed in large numbers, and dozens of other protesters have been detained across the country.
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TV pictures showed demonstrators chanting "Down with [Russian President Vladimir] Putin!", "Russia without Putin!" and "Putin is a thief!".
Correspondents say the marches appear to be the biggest since anti-government demonstrations in 2011/2012.
Navalny was detained as he arrived to join the rally in central Moscow. Protesters then tried to prevent a police van from taking him away.
In a tweet after his detention, he urged fellow protesters to continue with the demonstration.
"Guys, I'm fine. No need to fight to get me out. Walk along Tverskaya [Moscow main street]. Our topic of the day is the fight against corruption," he said (in Russian).
Navalny said staff at his office had also been detained for streaming the protests live online without permission.
Demonstrations were also held in Saint Petersburg, Vladivostok, Novosibirsk, Tomsk and several other cities, where arrests had also been reported.
Why the protests?
Alexei Navalny called for the nationwide protests after he published reports claiming that Mr Medvedev controlled mansions, yachts and vineyards - a fortune that far outstripped his official salary.
Mr Medvedev's spokeswoman called the allegations "propagandistic attacks", but the prime minister himself has not commented on the claims.
The reports included the accusation that Mr Medvedev had a special house for a duck on one of his properties - and on Sunday, some demonstrators held up images of yellow rubber ducks.
Others showed up with their faces painted green, a reference to a recent attack in which Mr Navalny was hit with green liquid.
In Moscow, protesters filled Pushkin square and some climbed the monument to poet Alexander Pushkin shouting "impeachment". Turnout was estimated to be between 7,000 and 8,000, according to police.
In the capital alone, at least 130 protesters had been taken by police, a rights group, OVD Info, was quoted by AFP news agency as saying.
State TV channels did not cover the demonstrations. Local media reports suggested the authorities pressured students not to attend.
In some cities, exams were scheduled on Sunday.
Alexei Navalny announced his intention to run for president in 2018 against Vladimir Putin. But he is barred from doing so after being found guilty in a case he said was politicised.
He said on his website that protests were planned in 99 cities, but that in 72 of them authorities did not give permission.