Protesters gathered outside the detention centre holding Sergei Tikhanovsky, husband of opposition leader Sviatlana Tsikhanouskaya, to celebrate his 42nd birthday.
The blogger was imprisoned by President Alexander Lukashenko ahead of the presidential election.
Footage filmed on August 18 shows crowds outside the jail in Minsk showing their support for Tikhanovsky.
The protesters are heard singing "Mury" a protest song sung by Polish singer Jacek Kaczmarski in 1978.