MOSCOW — Russia’s most prominent opposition figure, Aleksei A. Navalny, raised questions on Monday about whether he had been poisoned in prison after being convicted of calling for a protest that led to one of the largest street demonstrations in Moscow in years.

Mr. Navalny, 43, was sent from the government hospital where he was being treated for an unknown illness back to prison earlier in the day. His return to jail came over the strenuous objections of his doctor, who said that the cause of his symptoms had not been identified, but that he had apparently been poisoned with a “toxic agent.”

The opposition leader posted a message on his official website that addressed the possibility the authorities had poisoned him.

Mr. Navalny wrote that on the one hand, why would the authorities poison him while in custody, as they would be the obvious culprits? On the other hand, he noted, this had never stopped them before.