Alexei Navalny and more than 100 other demonstrators have been arrested at a rally in central Moscow, activists say.
The Russian opposition leader had urged protesters to gather in Manezh Square near the Kremlin after his brother, Oleg Navalny, was sentenced to jail.
Police detained Alexei Navalny at the protest and took him home, where he has been under house arrest since February.
Alexei Navalny 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.
Alexei 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.
At least 171 people were detained by police said OVD Info, a campaign group that monitors political detentions. However there was no official confirmation of the number of arrests.
Upon arriving at the rally, police detained Alexei Navalny 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, Alexei 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, Alexei Navalny 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.
Alexei Navalny has been under house arrest since February as part of a separate five-year suspended sentence for the alleged theft of 16 million rubles from a timber firm in 2009.
Vladimir Putin’s critic Alexei Navalny has avoided jail after getting a suspended sentence in a high-profile fraud case.
Alexei Navalny, 38, was given a suspended prison sentence of three-and-a-half years for defrauding two firms.
His brother, Oleg Navalny, is facing a three-and a-half year custodial sentence.
Alexei Navalny has consistently said the charges were politically motivated. He accused President Vladimir Putin’s government of targeting the relatives of his opponents.
The Navalny brothers were convicted of stealing 30 million rubles ($518,000) from the companies, one of which is an affiliate of French cosmetics giant Yves Rocher.
Prosecutors had demanded 10 years in prison for Alexei Navalny and 8 years for his brother Oleg. Although both were found guilty of embezzlement, only his younger brother will be jailed.
Alexei Navalny denounced the verdict as political “pressure”.
He tweeted: “Of all of the possible types of sentence, this is the meanest.”
Alexei Navalny was shocked by the outcome of the trial for his brother.
“Why are you putting him in prison?” he asked the judge.
“To punish me even harder?”
Alexei Navalny has been one of Vladimir Putin’s fiercest critics for several years, leading mass protests against his government in 2011 and 2012.
Tuesday’s verdict is the latest in a series of criminal cases against Alexei Navalny, which he says are fabricated to neutralize his political influence.
In 2013, Alexei Navalny came second in Moscow’s mayoral election, supported by 27% of voters.
He is already under house arrest, serving a 5-year suspended sentence for the alleged theft of 16 million rubles ($276,000) from a timber company in 2009.
The latest verdict was due to have been announced next month, but the court session was abruptly moved forward to December 30 after thousands of his supporters announced plans for a big protest rally on January 15.
After sentencing, Alexei Navalny called on his supporters to protest.
“I call on everyone to take to the streets today,” he said.
Thousands have already said on Facebook that they plan to attend a rally in Moscow’s Manezh Square, near the Kremlin, at 19:00 local time on December 30.