Syarhei Tsikhanouski, the husband of Belarus's opposition leader, has been sentenced to 18 years in jail for organising mass unrest and inciting social hatred.
Five supporters of Mr Tsikhanouski were tried along with him and sentenced to prison terms ranging from 14 to 16 years.
Mr Tsikhanouski, a video blogger, was jailed in May 2020 as he prepared to run against Belarus leader Alexander Lukashenko in a presidential election in August that year. He denied the charges.
His wife Sviatlana Tsikhanouskaya ran in his place in the election, which led to months of mass protests after Lukashenko claimed a landslide victory and the opposition accused him of rigging the ballot.
A few hours before the verdict was pronounced, Ms Tsikhanouskaya called it "illegal" and something that "should not be tolerated".
"Commenting on the so-called "verdict", I will ask myself only one question: what will I do with this news? And I will continue to defend the person I love, who has become a leader for millions of Belarusians," she said in a video message.
"I will try to do something very difficult, perhaps impossible, in order to bring closer the moment when we will see him in the new Belarus."
Ms Tsikhanouskaya fled to neighbouring Lithuania to escape a sweeping crackdown after the election. She has since gained prominence, meeting an array of western leaders.
Mr Tsikhanouski's trial was closed to the public and lawyers were banned from disclosing details of the case.
In July, a Belarusian court jailed former presidential contender Viktor Babariko for 14 years after convicting him on corruption charges he denied.
In September, Maria Kolesnikova, one of the leaders of mass street protests against Mr Lukashenko last year, was sentenced to 11 years in prison.
UN Special Rapporteur Anaïs Marin said more than 35,000 people had been arbitrarily detained over the past year and that the fear of repression had caused tens of thousands of Belarusians to flee to seek refuge abroad.
The European Union has condemned the court's decision to sentence opposition Mr Tsikhanouski and other activists to "harsh and unfounded" prison terms.
"These sentences are part of the ongoing brutal and systematic repression of all independent voices in Belarus," EU foreign affairs spokesman Peter Stano said.