The jailed leader of the unregistered Young Front opposition organization, Zmitser Dashkevich, has received one additional year in prison.
An in camera trial hearing within the penitentiary in the eastern town of Hlybokaye on August 28 found Dashkevich guilty of a "systematic and aggressive refusal to follow instructions of the penitentiary administration.
The human rights watchdog Amnesty International has recognized Dashkevich as a "prisoner of conscience," jailed for his political views.
Last month, Amnesty urged Belarusian authorities to "stop the harassment" of Dashkevich.
Dashkevich was sentenced to two years in jail after being found guilty in March 2011 of assaulting two people in Minsk in December 2010, one day before Belarus's disputed presidential election, which was followed by mass arrests of activists.
He pleaded not guilty to the charges.