Belarus dissident Vasily Parfenkov has been jailed for four years after a mass protest against the re-election of President Alexander Lukashenko.
Mr Parfenkov, 28, was convicted of breaking a window at the parliament building during the December unrest.
He had campaigned for opposition candidate Vladimir Neklyayev in the 19 December election and was the first activist to go on trial.
Five ex-candidates are among 42 people charged with inciting mass disorder.
Thousands of opposition activists gathered in central Minsk as the polls closed that day. They were later violently dispersed by riot police.
Mr Parfenkov told the judge that he attended the mass protest but did not smash any windows in the government building, the Belapan news agency reported from Minsk.
The prosecution alleged that he and others caused damage to the building amounting to $4,670 (3,440 euros, ?2,900) and had asked for a six-year sentence.
Independent observers said election procedures were flawed.
Hundreds of people were detained in the hours after the demonstration including opposition candidates Vladimir Neklyayev and Andrei Sannikov. Although Mr Neklyayev has been provisionally released under house arrest, Mr Sannikov is still being held.
The authorities' crackdown also involved numerous searches of activists' and journalists' homes, where computers and other items were seized.
In a separate development, the Belarusian justice ministry announced it had revoked the licences of four lawyers acting on behalf of the activists. The ministry said two of the lawyers had refused to continue defending Mr Sannikov's wife, journalist Irina Khalip, according to the Belpan news agency.
The family of Ms Khalip, who was among those arrested after the protests, said the two lawyers had pulled out of the case after coming under pressure from the authorities, the agency said.
The US and the European Union have imposed sanctions on Belarus over the arrests.
Mr Lukashenko and more than 150 other officials are barred from visiting the 27 EU countries, and a list of officials with assets frozen by the EU has been extended.
The EU had originally barred Mr Lukashenko from visiting in 2006, after the last presidential election, but suspended the ban two years later in an attempt to encourage reforms.
Mr Lukashenko has been in power in the ex-Soviet republic since 1994, using the security forces and censorship to stifle dissent.