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.
Young Russians get Navalny message: Sarah Rainsford, BBC News, Moscow
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.
Read more from Sarah: Why Navalny and protesters rattle Kremlin
What happened in court?
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.
What were the protests about?
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.
What have the EU and US said?
An EU spokesman said the Russian police action had "prevented the exercise of basic freedoms of expression, association and peaceful assembly, which are fundamental rights enshrined in the Russian constitution".
The statement added: "We call on the Russian authorities to abide fully by the international commitments it has made... and to release without delay the peaceful demonstrators that have been detained."
US state department spokesman Mark Toner said in a statement: "The Russian people, like people everywhere, deserve a government that supports an open marketplace of ideas, transparent and accountable governance, equal treatment under the law, and the ability to exercise their rights without fear of retribution."
What have the media said?
Russian state television completely ignored the protests on Sunday. Monday morning's bulletins were similarly blank.
Pro-Kremlin newspapers also ignored the protests.
But the BBC's Steve Rosenberg in Moscow says there was coverage in some others. Business daily Vedomosti reported a high level of dissatisfaction with the authorities, saying that the young generation had become politicised.
Another paper refers to "Putin's disastrous anniversary" - the protests coming 17 years after the president took office - and says: "A few months ago, Alexei Navalny was seen as yesterday's man who'd missed the bus. He hasn't missed any bus."
Who is Alexei Navalny?
The most prominent critic of President Putin, Mr Navalny began his anti-corruption campaign with blogs aimed at state-controlled companies in 2008.
He moved on to opposing the ruling party, United Russia, calling it the "party of crooks and thieves".
He led massive protests following the 2011 election, the biggest in Moscow in December that year, after which he was arrested and jailed for 15 days.
He has said he will run for president in 2018, but a court has convicted him of embezzlement, which would bar him. He denies the charges, calling the case farcical.