A court on Monday jailed for 15 days Russian opposition leader Alexei Navalny for his role in organising what authorities said was an illegal protest in Moscow on Sunday.
The court also fined him 20,000 roubles ($352).
Tens of thousands of protesters took to the streets throughout Russia on Sunday, many calling on Prime Minister Dmitry Medvedev to resign.
It was the biggest show of defiance since anti-government protests in 2011. Protests were held in major cities across the country.
Russian police say that about 500 people were arrested nationwide. Human rights groups say 1,000 were taken into custody.
The protests were led by Navalny, an opposition leader who recently announced his intention to run against Vladimir Putin in the 2018 presidential election.
Police detained Navalny in Moscow as he walked along central Moscow's Tverskaya Street on Sunday with supporters.
Navalny is arguably Russia's most popular opposition leader. He has been twice convicted on fraud and embezzlement charges that he has dismissed as politically motivated. He is currently serving a suspended sentence. His supporters say Sunday's arrest could be used as a pretext to convert it into jail time.
The same court was due, later on Monday, to consider a separate charge against him of disobeying a police officer.
The Kremlin has dismissed the opposition as Westernised urban elite disconnected from the issues faced by poor Russians.
TRT World has the latest from Moscow.
EU seeks release 'without delay'
The European Union urged Russia "to release without delay" protestors detained during the Sunday rally.
An EU spokesman 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.
But the Kremlin rejected the call for release, accusing the opposition of paying teenagers to attend the protest.
Kremlin spokesman Dmitry Peskov called the protests "a provocation." He said the authorities were concerned that opposition activists would try to encourage people to break the law again in the future.