Pyotr Verzilov, a member of the Russian dissident art troupe Voina and an unofficial spokesman for the Pussy Riot protest group, is said to be in the hospital in serious condition, and friends say they fear he has been poisoned.
The Meduza website on September 12 quoted Verzilov's partner, Veronika Nikulshina, as saying he had begun to lose his sight, speech, and mobility.
The website quoted friends as saying that Verzilov, founder of the Mediazona website, is being treated in the toxicology section of Moscow's Bakhrushin City Clinical Hospital.
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Mediazona, which reports on trials of Russian activists, also said that Verzilov was hospitalized in serious condition.
"Our friend, brother, comrade Petr Verzilov is in reanimation. His life is in danger. We think that he was poisoned," Pussy Riot wrote on its official Twitter account with a link to an article about the hospitalization.
Ottawa said that Verzilov is a Canadian national.
"We are certainly taking this very seriously," Canadian Prime Minister Justin Trudeau told reporters. "But it is too early to draw any conclusions about what has happened or how it happened.
"We just know that there is a Canadian who is in hospital that we need to make sure we are getting support to," he added.
Earlier this year, Verzilov was sentenced along with Pussy Riot members to 15 days in jail for briefly interrupting the July 15 World Cup final in Moscow between France and Croatia by running onto the pitch wearing fake police uniforms.
Verzilov became known as a member of the dissident art group Voina (War) in the late 2000s.
He performed with this then-wife, Nadezhda Tolokonnikova, who went on to form punk protest band Pussy Riot with Maria Alyokhina and Yekaterina Samutsevich.
Pussy Riot members came to prominence after they were convicted of "hooliganism motivated by religious hatred" for a stunt in which they burst into Moscow's Christ the Savior Cathedral and sang a "punk prayer" against Vladimir Putin, who was prime minister and campaigning for his return to the presidency at the time.
Alyokhina and bandmate Tolokonnikova were close to the end of their two-year prison sentences when they were freed in December 2013, under an amnesty they dismissed as a propaganda stunt to improve Putin's image ahead of the 2014 Sochi Winter Olympics.
Alyokhina and Tolokonnikova founded Mediazona in 2014, with Verzilov becoming publisher.