Human Rights Watch said Bialiatski was subjected to harsh treatment while in prison. He was arrested on tax evasion charges in 2011 and was sentenced later to more than four years in prison.
"I call upon the authorities to immediately and unconditionally release and rehabilitate Mr. Bialiatski and those incarcerated in violation of their human rights," Miklos Haraszti, a U.N. special envoy on human rights issues, said.
Bialiatski served as director of a human rights center in Belarus. The United Nations said he was prevented from working because authorities in Minsk wouldn't register his group as a non-governmental organization.
The Organization for Security and Cooperation said there was a general lack of political freedom in the country. Several opponents of Belarusian President Alexander Lukashenko were imprisoned following December 2010 elections.
"Bialiatski's detention is a symbol of the repression against human rights defenders," Haraszti said in a statement Friday.