Russian opposition leader Alexei Navalny has been jailed for 15 days for resisting police orders during mass protests on Sunday.
Mr Navalny was among several hundred people who were detained in connection with the rallies across the country.
The court in Moscow earlier fined him the minimum 20,000 roubles ($350) for organising the banned protests.
On Monday the Kremlin accused the opposition of encouraging lawbreaking and provoking violence.
Some young people were paid to attend, a presidential spokesman said.
Mr Navalny later repeated accusations of corruption against Prime Minister Dmitry Medvedev.
The allegations were the main reason behind Sunday's protests, which drew thousands of demonstrators nationwide, including in St Petersburg, Vladivostok, Novosibirsk, Tomsk and several other cities, as well as Moscow.
While Moscow police reported earlier that 500 people had been detained, the OVD-Info human rights group said 1,030 people had been detained in Moscow and that at least 120 of them were still being held.
Perhaps the most striking thing about the protests that swept Russia this weekend was the make-up of the crowd which included significant numbers of young people, even schoolchildren.
The Kremlin's spokesman has criticised organisers for luring them onto the streets, claiming they were paid to turn out. Silent until he spoke, state TV channels have picked up that suggestion and run with it.
But these were young Russians who have grown up under Vladimir Putin on a diet of patriotism. On Sunday, they joined a crowd chanting "Putin - thief" and "Russia will be free!"
Their presence is proof of how effectively Alexei Navalny and his team use social media.
Mr Navalny appeared in court after being detained on Sunday and spending the night in jail. Although he escaped a jail sentence on the first charge, he was given 15 days for disobeying a police officer.
Mr Navalny's lawyer, Olga Mikhailova, told Reuters news agency she had expected such a verdict and would appeal against it.
Before his appearance on Monday, Mr Navalny, 40, tweeted from the building: "Hello everyone from Tverskoy Court. The time will come when we will have them on trial (only honestly)."
He argued it was Mr Medvedev who should be summoned as the chief organiser of the protests, because his "corrupt activities led to people coming on to the streets of 99 Russian cities".
Mr Navalny, denying all the charges, said: "They haven't heard witnesses, nor have they satisfied any of our requests. Even the slightest semblance of justice is totally absent here."
He also said again that he plans to run for president in 2018.
Mr Navalny called for the nationwide protests after he published reports claiming that Mr Medvedev controlled mansions, yachts and vineyards - a fortune that suggests income that far outstrips his official salary.
His report, posted on YouTube, has been viewed more than 12 million times.
It includes 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 against him "propagandistic attacks".
Many young people including teenagers took part in Sunday's protests, prompting one report to speak of the rise of the "YouTube generation" as players in the opposition movement.
Alexei Navalny's use of social media symbolises his political style, reaching out to young followers in sharp, punchy language, mocking the establishment loyal to President Vladimir Putin.
