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.
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.