Navalny, by far Russia’s most famous opposition leader, rose to prominence more than a decade ago by lampooning the elite class round President Vladimir Putin and voicing allegations of corruption on a vast scale. He was 47.

The Federal Penitentiary Service of the Yamalo-Nenets Autonomous District said in a statement that Navalny “felt unwell” after a walk at the IK-3 penal colony in Kharp, about 1,900 km (1,200 miles) north east of Moscow.

Navalny, the prison service said, had lost consciousness almost immediately.

“The medical staff of the institution arrived immediately, and an ambulance team was called,” the prison service said.

“All necessary resuscitation measures were carried out, which did not yield positive results. Doctors of the ambulance stated the death of the convict.”

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“The causes of death are being established.”

Putin has been told about Navalny’s death, Kremlin spokesman Dmitry Peskov said.

Navalny earned admiration from Russia’s disparate opposition for voluntarily returning to Russia in 2021 from Germany, where he had been treated for what Western laboratory tests showed was an attempt to poison him with a nerve agent.

Navalny said at the time that he was poisoned in Siberia in August 2020. The Kremlin denied trying to kill him and said there was no evidence he was poisoned with a nerve agent.