Maria Kolesnikova, who lead mass street protests against Belarusian President Alexander Lukashenko last year, was sentenced to 11 years in jail on Monday (September 6).
The 39-year-old was detained after ripping up her passport, an attempt to stop Belarusian security forces from deporting her in a standoff at the Ukrainian border in September.
She and another senior opposition figure, Maxim Znak, were charged with extremism and trying to seize power illegally.
They both deny wrongdoing, with Kolesnikova calling the charges "absurd."
Znak was sentenced to 10 years in prison.
The pair were held in glass cages ahead of the verdict.
Kolesnikova raised her handcuffed hands to make her trademark heart sign and smiled for the cameras.
The musician-turned-politician became one of the faces of a large opposition movement during the country's presidential election in August 2020.
Protesters claim the vote was rigged in an effort to prolong Lukashenko's grip on power.
She is one of three women, all political novices, who joined forces to front the election campaign against him, after higher-profile male candidates were barred from standing.
Lukashenko, who vehemently denies electoral fraud, has been in office since 1994.
He has faced fresh Western sanctions in recent months, after launching a violent crackdown on his opponents.
The trial, which began last month, was closed to the public on national security grounds.
The European Union denounced the verdict, while Britain's foreign minister called it an assault on defenders of democracy.
The circumstances of the case, the investigators and the witnesses have not been disclosed.