It said that Pyotr Verzilov was poisoned, reports the Guardian.
"Our friend, brother, comrade Petr (Pyotr) Verzilov is in reanimation," Pussy Riot tweeted, using the common Russian translation for intensive care.
"His life is in danger. We think that he was poisoned."
Verzilov reportedly fell ill after he attended a court hearing with Veronika Nikulshina, a member of Pussy Riot, on Wednesday.
Over a period of two hours "he got worse exponentially", Nikulshina told Russian news outlet Meduza later in the night.
"First his sight, then his ability to speak, then even his ability to walk," she added.
Verzilov was one of four members of the group to invade the pitch, dressed in a police officer's uniform, during the World Cup final between France and Croatia on July 15.
It was part of a protest against Russian police brutality after a series of torture allegations were reported from prisons and police interrogation rooms across the country, the Guardian said.
Verzilov has been involved in protest art in Russia for more than a decade, participating in both the Voina art collective and the formation of Pussy Riot.
He also became the publisher of Mediazona, an independent Russian language site which focuses on court cases and conditions in Russian prisons.
(This story has not been edited by Social News XYZ staff and is auto-generated from a syndicated feed.)
