Top Kremlin critic Alexei Navalny appeared in court on Monday after he and more than 1,000 other people were arrested at an anti-corruption protest in Moscow.
The United States and the European Union voiced deep concern about the detentions, with the State Department describing them as an "affront to democracy".
Navalny had called for the protests that swept the country Sunday after he published a report accusing Prime Minister Dmitry Medvedev of controlling a property empire through a murky network of non-profit organisations.
A haggard-looking Navalny arrived at a Moscow district court in a police van, posting on Twitter a selfie taken in the courtroom.
"The time will come when we will put them on trial (but fairly this time)," he wrote on Twitter ahead of the proceedings.
About 7,000 to 8,000 people demonstrated in the heart of the Russian capital on Sunday, according to police, making it one of the biggest unauthorised rallies in recent years.
Demonstrations were held not just in Moscow and Russia's second city Saint Petersburg but also in a number of provincial cities where protests are rarely seen.
They attracted a significant number of minors born during President Vladimir Putin's 17 years in power.
Free Protesters 'Without Delay'!
Navalny, who has announced plans to run for president in the 2018 election, was arrested as he was walking to the Moscow protest.
He could face up to 15 days in police custody for having called for unsanctioned protests, his spokeswoman Kira Yarmysh wrote on Twitter.
Protests swept Russia on Sunday after a report was published accusing Prime Minister Dmitry Medvedev of controlling a property empire through a murky network of nonprofit organisations (Olga Maltseva / AFP).
About 1,030 people were arrested at the Moscow rally, according OVD-Info, a website that monitors the detention of activists.
The vast majority were released overnight after being fined, while about 120 remained in police custody on Monday, OVD-Info said. One policeman was hospitalised after suffering a head injury during the Moscow rally, the interior ministry said.
The European Union urged Russia to release the demonstrators "without delay". An EU spokesperson said the police action had "prevented the exercise of basic freedoms of expression, association and peaceful assembly — which are fundamental rights enshrined in the Russian constitution."
"We call on the Russian authorities to abide fully by the international commitments it has made, including in the Council of Europe… to uphold these rights and to release without delay the peaceful demonstrators that have been detained."
US State Department spokesman Mark Toner said the detention of "peaceful protesters, human rights observers, and journalists is an affront to core democratic values".
'Hope For a Normal Future'
Russian opposition leader Alexei Navalny has been the subject of several legal cases in recent years (Thomas Saint-Cricq / AFP)
The protests were reminiscent of the mass anti-government rallies that swept Russia in 2011 over vote-rigging after a parliamentary election, which snowballed into the biggest challenge against Putin since he took power in 2000.
Navalny said Sunday that he was "proud" of the demonstrators who had taken to the streets.
"You are the country's best people and Russia's hope for a normal future," he wrote on Twitter.
Despite the large scale of the protests, Russian state television news did not cover them, broadcasting soap operas and nature films instead.
Pro-Kremlin television host Vladimir Solovyov accused Navalny during a talk show on Sunday of being a "paid provocateur" seeking to "destroy" the country.
The Russian constitution allows public gatherings, but recent laws have criminalised protests unauthorised by city authorities, which frequently refuse to grant permission for rallies by Kremlin critics.
Navalny, a 40-year-old lawyer by training, first announced plans to run for the presidency after he won a surprise 27 percent of the vote in the Moscow mayoral election in 2013.
But he has been the subject of several legal prosecutions in recent years, and in February he was found guilty of embezzlement in a case he has condemned as politically motivated and given a five-year suspended sentence which could make him ineligible to run in next year's vote.
