Russia's main opposition leader, Alexei Navalny, has been arrested at an anti-corruption protest he organised in the capital, Moscow.
Thousands of people 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.
In Moscow, protesters filled Pushkin square and some climbed the monument to poet Alexander Pushkin shouting "impeachment". Turnout was estimated to be between 7000 and 8000, police said.
They said 500 protesters had been arrested in the capital alone, but rights group OVD Info put the number at at least 700.
Hundreds more were arrested in Saint Petersburg, Vladivostok, Novosibirsk, Tomsk and several other cities, in what correspondents said were the biggest protests since anti-government demonstrations in 2011/2012.
TV pictures showed demonstrators chanting "Down with [Russian President Vladimir] Putin!", "Russia without Putin!" and "Putin is a thief!".
Mr 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).
He had 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.
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.
Mr Medvedev's spokeswoman called the allegations "propagandistic attacks", but the Prime Minister himself has not commented on the claims.
Mr Navalny later said police had stormed the office of his foundation and detained its staff, who were broadcasting the protests live.
The Kremlin has not commented on the demonstrations. It said on Friday that plans for an unauthorised protest in central Moscow were an illegal provocation.
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.