Russian anti-Kremlin activist Alexei Navalny has been found guilty of fraud and given a suspended three-and-a-half-year jail sentence.
His younger brother Oleg, who has no role in the opposition movement, was sent to prison for the same period in a case seen as part of a campaign to stifle dissent.
The verdict was scheduled for next month, but was abruptly moved forward to the day before New Year's Eve, the main holiday in Russia, leading to speculation that authorities wanted to head off planned protests.
More than 30,000 people had previously taken to Facebook to say they would join a rally outside the Kremlin to protest about what they called an unlawful trial.
The Navalny brothers were accused of stealing 30 million roubles, around $500,000 (£372,000) at the current exchange rate, from two firms, including an affiliate of the French cosmetics company Yves Rocher between 2008 and 2012.
The brothers were also each fined 500,000 roubles (about £5,685) and ordered to pay some four million roubles (£45,500) in damages.
Prosecutors asked had asked that Alexei Navalny be imprisoned for 10 years.
The imprisonment of Oleg Navalny, the father of two small children and a former executive of the state-owned postal service, could echo the Soviet-era practice of punishing the relatives of inconvenient people.
"Aren't you ashamed of what you're doing? You want to punish me even harder?" Alexei Navalny shouted out as Judge Yelena Korobchenko handed down the sentence for his brother.
Alexei briefly entered the metal cage that his brother was put into after the verdict and appeared to be holding back tears.
"This is the most disgusting and vile of all possible verdicts," Alexei Navalny said outside the court.
"The government isn't just trying to jail its political opponents - we're used to it, we're aware that they're doing it - but this time they're destroying and torturing the families of the people who oppose them."
The suspended sentence means could be converted into a prison term if Alexei Navalny offends again, although this will be still up to the court to decide.
He has been under house arrest since February and his lawyer Vadim Kobzev says he will remain there until all appeals are exhausted, which could take months.
The trial itself seemed to be full of inconsistencies and loopholes.
Yves Rocher complained to investigators but its representatives have insisted throughout the trial that there never were any damages.
The prosecutors insisted that the brothers forced the company "into disadvantageous contracts" and defrauded it of 26 million roubles (nearly £300,000).
Navalny, a lawyer and popular blogger, rose to prominence with his investigations of official corruption and played a leading role in organising massive anti-Putin demonstrations in Moscow in 2011 and 2012.
In a 2013 trial in a different criminal case, he was found guilty of embezzlement and sentenced to prison, but he was released the next day after thousands of people protested in the streets of Moscow.
He was then handed a suspended sentence and finished a strong second in Moscow's mayoral election in September 2013.
