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.
At least 500 other protesters were detained in the capital and across the country.
Most of the marches were illegal, organised without official permission.
Hundreds of other protesters have been detained across the country, activists say.
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.
Alexei 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 later said police stormed the office of his foundation and detained its staff, who were broadcasting the protests live.
Demonstrations were also held in Saint Petersburg, Vladivostok, Novosibirsk, Tomsk and several other cities, where arrests had also been reported.
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.
The police said 500 protesters had been arrested in the capital alone, but a rights group, OVD Info, put that number at at least 700.
The Kremlin has not commented on the demonstrations. It had said on Friday that plans for an unauthorised protest in central Moscow were an illegal provocation.