Maksim Vinyarski of the European Belarus movement was found guilty on March 26 of chanting extremist slogans and using vulgar words.
He was arrested along with about a dozen other activists on March 25 when more than 1,500 protesters marched through downtown Minsk in an annual opposition rally.
The march -- called Freedom Day -- marks the anniversary of the short-lived Belarusian Popular Republic in 1919.
This year's rally attracted the largest public crowds in Belarus since protests against the country's disputed presidential election in 2011.
Hundreds were arrested then.
Protesters at the march carried Ukrainian and Belarusian flags and held posters saying "Russia Means War!", "Glory To Ukraine!", and "Death To The Kremlin Occupants!"
Almost all of the arrested activists were released hours later..