Russian opposition leader Alexei Navalny and more than 100 other demonstrators have been arrested at a rally in central Moscow, activists say.
Navalny had urged protesters to gather in Manezh Square near the Kremlin after his brother was sentenced to jail.
Police detained Navalny at the protest and took him home, where he has been under house arrest since February.
He says the legal cases against him and his brother are motivated by his opposition to President Vladimir Putin.
Hours earlier, a court in Moscow handed Alexei Navalny a suspended prison sentence of three-and-a-half years for defrauding two firms. His brother Oleg was given a three-and-a-half-year custodial sentence for the same offence.
Navalny accused authorities of trying to punish him by jailing his brother, and called for his supporters to gather in central Moscow in protest.
Around 2,000 people gathered in temperatures below -10C.
Pro-government protesters are staging a counter-demonstration nearby, our correspondent Steve Rosenberg reports.
More than 200 people were detained at the rally in support of Navalny while others were pushed back by the police into the metro stations
But a few hours later dozens of them, including well-known Russian bloggers, members of the group Pussy Riot and other opposition activists, climbed into a huge Christmas ball, a street decoration a few steps away from the Kremlin wall.
They continued their protest through the night as the temperature dipped to minus 20C.
'Take to the streets'
After the sentencing of his brother on Tuesday morning, a visibly shocked Navalny called on his supporters to "take to the streets".
Upon arriving at the rally, police detained the opposition leader and drove him to his house in a police van.
He said he was not allowed out of his flat and that five soldiers stood guard outside.
After his arrest, Navalny sent a tweet saying that he "had not got as far as the square".
"I call on everyone not to leave until they are forced to," he said. "They cannot arrest everyone".
Just before his arrest, he told reporters that his motivation was "not my brother, my family, myself or some specific people" but "this disgusting outrage that has been going for many years".
Prosecutors had demanded 10 years in prison for Alexei Navalny and eight years for his brother Oleg. One prominent MP from President Putin's United Russia party said they should launch an appeal for a stronger sentence.
In court - Steve Rosenberg, Moscow correspondent
Alexei Navalny is the opposition figure the Kremlin fears most. The case against him was widely seen as politically motivated.
And because his brother was also on trial he had accused the authorities of trying to take his family members hostage to get at him.
Alexei was visibly shocked by the sentence. "Why are you jailing him [Oleg]?" he shouted at the judge. "Just to punish me even harder?"
The US said it was "troubled" by the guilty verdict and called it a "disturbing development".
Alexei Navalny has been under house arrest since February as part of a separate five-year suspended sentence for the alleged theft of 16m roubles from a timber firm in 2009.
He is the most high-profile opposition figure in Russia and came second in Moscow's mayoral election last year, with 27% of the vote.