Navalny is Russian President Vladimir Putin's most visible and adamant opponent.
His personal physicians have not been allowed to see him in prison. He went on hunger strike to protest the refusal to let them visit when he began experiencing severe back pain and a loss of feeling in his legs.
Russia's state penitentiary service has said that Navalny is receiving all the medical help he needs.
Navalny was arrested on January 17 when he returned to Russia from Germany, where had spent five months recovering from Soviet nerve-agent poisoning that he blames on the Kremlin. Russian officials have denied any involvement and even questioned whether Navalny was poisoned, which was confirmed by several European laboratories.
Asked about Navalny's worsening condition, US President Joe Biden told reporters today: "It's totally totally unfair and totally inappropriate. On the basis of having the poison and then on a hunger strike."
Navalny was ordered to serve 2.5 years in prison on the grounds that his long recovery in Germany violated a suspended sentence he had been given for a fraud conviction in a case that Navalny says was politically motivated.