Dmitry Arganovsky, a lawyer for opposition backer Vladimir Akimenkov, said his client isn't getting the medical care he needs to treat a congenital eye condition.
"It's clear when we see him that his vision is worsening," he told state-funded broadcaster RIA Novosti. "He can barely see the documents we give him."
A Moscow court ruled Monday that Akimenkov was in no medical danger while in prison.
Akimenkov was arrested in May following clashes that coincided with the inauguration of Vladimir Putin to a third non-consecutive term as president of Russia. He's accused of assaulting police officers during the demonstrations but denies the charges.
Several opposition supporters have been detained in the country amid growing frustration with Putin's leadership. The Russian news service reports that one person of the 17 formally arrested and charged since May have been sentenced.
Sergei Udaltsov, leader of the opposition Left Front movement, is under investigation for allegedly discussing revolutionary ambitions with political leaders in Georgia. He was quoted by RIA Novosti as comparing the Russian courts to the Nazi secret police when asked about the Akimenkov ruling.