Dzemidzenka's wife told RFE/RL on August 30 that her husband was sentenced late on the previous day.
Police detained Dzemidzenka earlier on August 29 while he was on his way to a press conference held by the recently-released leader of the Young Front, Zmitser Dashkevich.
Dashkevich was released from prison in the western city of Hrodna on August 28 after serving almost three years in jail.
Dashkevich and fellow Youth Front activist Eduard Lobau were found guilty in March 2011 of assaulting two people in Minsk a day before the disputed presidential election in December 2010.
Lobau was sentenced to four years in prison. They say the charges against them were fabricated.