Pyotr Verzilov, a Pussy Riot activist whose demonstrations against the Russian government include running onto the field during the World Cup final, has been hospitalized in Moscow in what the protest group suspects was a poisoning attack, according to local news media reports.

Veronika Nikulshina, a fellow Pussy Riot member, told the Meduza website that Mr. Verzilov, 30, lost his sight, speech and mobility. He was being treated Wednesday in the toxicology wing of a hospital, she said.

She said he started feeling ill shortly after a court hearing on Tuesday and lay down to rest at 6 p.m. Two hours later, he told Ms. Nikulshina, whom the newspaper described as his partner, that he was starting to lose his sight.

Paramedics arrived as his condition rapidly worsened, and he told them he had not eaten anything or taken drugs. He started convulsing, and in the ambulance began “babbling,” Ms. Nikulshina said.