Vyacheslav Ivanets, who said he visited Leonid Razvozzhayev at the center on January 15, told RFE/RL that guards do not allow Razvozzhayev to sit or lie on his bed between 6 a.m. and 10 p.m.
Ivanets says Razvozzhayev also complained that he has health problems and cannot eat the jail's food.
He also says Razvozzhayev is banned from buying food and newspapers from a shop inside the detention center.
Razvozzhayev was arrested in October for his alleged role in organizing unsanctioned, violent protests in Moscow.
He is also being charged with armed robbery in connection with a 15-year-old incident.
Razvozzhayev says authorities kidnapped him and used torture to force him to confess.