MOSCOW — Alexei Navalny, Russia’s most prominent political opposition figure, has died in a remote Russian prison at age 47.

News of Navalny’s death came Friday from the Federal Penitentiary Service in the Yamalo-Nenets Autonomous District, above the Arctic Circle.

In a statement, prison authorities said Navalny “felt unwell” after a walk in the prison yard and soon lost consciousness. Attempts by emergency medics to resuscitate him “failed to give positive results.”

Navalny had been serving out a lengthy prison sentence for charges including extremism, which were widely seen as punishment for his years of criticism of Russian President Vladimir Putin.

Kremlin spokesperson Dmitry Peskov said Putin has been informed of Navalny’s death. He said prison medics are working to identify the cause of death.

Reactions swiftly poured in from around the world. U.S. Secretary of State Antony Blinken said Navalny’s “death in a Russian prison and the fixation and fear of one man only underscores the weakness and rot at the heart of the system that Putin has built. Russia is responsible for this.”

Concerns over Navalny’s well-being and safety while in prison were long-standing.

His family members and supporters say authorities repeatedly denied Navalny medical care and subjected him to long, punishing stints in solitary confinement with the apparent aim of preventing his access to the outside world. A representative of his Anti-Corruption Foundation in Washington, D.C., expressed the belief in April that Navalny was slowly being poisoned in prison.

Yet a video shared by Russia’s independent SOTA news service appeared to show Navalny looking healthy and in good spirits during a court hearing the day before his death. Navalny was participating by video feed.

Navalny had been serving out a 19-year prison sentence on charges including extremism, embezzlement and fraud — widely seen as Kremlin retribution for his political activities.