The main rival to Belarus's authoritarian president was sentenced to 14 years in prison, the Associated Press reported.
The U.S. Embassy called the verdict a "cruel sham" and said it showed that President Alexander Lukashenko's "regime will stop at nothing to keep power."
Viktor Babariko, the head of a commercial bank owned by Russian natural gas company Gazprom, hoped to challenged Lukashenko last year but was arrested before the election and was prohibited from registering as a candidate.
He was convicted on Tuesday on counts of bribery and money laundering. He denied the claims, saying the charges were politically motivated.
For more reporting from the Associated Press, see below.
At the time, he was widely perceived as Lukashenko's main rival, and his arrest drew thousands of protesters to the streets.
Babariko, who served as the chief executive of Belgazprombank, has remained jailed since his arrest.
"I can't plead guilty to the crimes I didn't commit," Babariko told the court before it issued the verdict. "I can tell you from that cage that I don't feel ashamed for my life before the people I knew."
Lukashenko's reelection to a sixth term was widely seen as rigged and triggered months of protests, the largest of which drew up to 200,000 people. Authorities responded to the demonstrations with a massive crackdown that saw more than 35,000 people arrested, thousands beaten by police and opposition leaders jailed or forced to leave the country.
Sviatlana Tsikhanouskaya, who ended up as the main opposition candidate, denounced what she described as a "mad sentence to the person who decided to enter politics and became one of the leaders who woke the country up from a long sleep."
"The regime is doing all that in order to kill any thoughts even distantly resembling faith and hope," said Tsikhanouskaya, who was forced to leave Belarus under official pressure after the election. "But the hope in our hearts is something that is the most precious for Viktor and thousands of other innocent people in prisons."
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Newsweek is committed to challenging conventional wisdom and finding connections in the search for common ground.