Maria Kolesnikova, one of the leaders of mass street protests against Belarusian President Alexander Lukashenko last year, was sentenced to 11 years in prison on Monday.
She was detained after ripping up her passport to prevent Belarusian security forces from deporting her in a standoff at the Ukrainian border in September.
Kolesnikova and another senior opposition figure, Maxim Znak, were charged with extremism and trying to seize power illegally. Both denied wrongdoing and Kolesnikova called the charges absurd.
Znak was sentenced to 10 years in prison.