Pyotr Verzilov, a Russian activist known for working with the protest group Pussy Riot, has been taken to hospital in a serious condition.
Friends and relatives suspect that Mr Verzilov has been poisoned, according to Meduza, a Russian website.
He fell ill after a court hearing for Veronika Nikulshina, a fellow Pussy Riot member.
Ms Nikulshina, said that Mr Verzilov woke up and realised that he was losing his sight.
"First it was his vision, then his ability to speak, and then his ability to walk,” she told Meduza.
Mr Verzilov was taken to a hospital in Moscow's Sokolniki district.
Ms Nikulshina said that Mr Verzilov was taken to the hospital's toxicology wing and that doctors told her that his condition was serious.
Mr Verzilov's mother was reportedly not allowed to see her son and was not given information about his condition.
Verzilov was married to Nadezhda Tolokonnikova, a Pussy Riot member who was jailed for two years in August 2012 after performing what the band called a "punk prayer" at a Moscow cathedral.
The performance was in protest against Vladimir Putin.
Mr Vezilov was one of a group of Pussy Riot activists who disrupted the World Cup final in July by storming the pitch.
Ms Tolokonnikova tweeted about Mr Verzilov's condition.
"Pyotr may have been poisoned. In the toxicology department in a very serious condition," she said.
There has been no official statement about Mr Verzilov's illness or about what caused it.
