Russia’s most prominent opposition leader Alexei Navalny collapsed and died on Friday after a walk at the “Polar Wolf” Arctic penal colony where he was serving a long jail term, the Russian prison service said.

Navalny, a 47-year-old former lawyer, rose to prominence more than a decade ago with blogs on what he said was vast corruption and opulence among the “crooks and thieves” of Russia’s elite, Reuters reported.

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 into the Arctic Circle.

He lost consciousness almost immediately, it said.

“All necessary resuscitation measures were carried out, which did not yield positive results,” the prison service said, adding that causes of death were being established.

The Kremlin said President Vladimir Putin was told about the death, which brought a torrent of outrage from the West, some saying the Russian leader bore responsibility.

Supporters of Navalny said they could not confirm he was dead, but that if he was then they believed he had been killed.

“Russian authorities publish a confession that they killed Alexei Navalny in prison,” Navalny aide Leonid Volkov wrote on social media.

Western officials paid tribute to his courage as a fighter for freedom. Some, without citing evidence, bluntly accused the Kremlin.

“Alexei Navalny paid with his life for his resistance to a system of oppression,” France’s Foreign Minister Stephane Sejourne said. “His death in a penal colony reminds us of the reality of Vladimir Putin’s regime.”

Navalny’s lawyer was on his way to the prison in Kharp where his client was serving sentences totalling more than 30 years, read the report.

Russian state television showed a press conference by the central bank chief as the news broke.

Navalny’s spokesperson Kira Yarmysh said she had no confirmation that he was dead.

“My sincere belief is that it was the conditions of detention that led to Navalny’s death,” Russian newspaper editor and Nobel Peace Prize laureate Dmitry Muratov told Reuters. “His sentence was supplemented by murder.”

Supporters had cast Navalny as a future leader of Russia who would one day walk free from jail to take the presidency, though many opposition activists had expressed fears that he was in grave danger in the Russian prison system.

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.

Navalny long forecast Russia could face seismic political turmoil, including revolution, because he said Putin built a brittle system of personal rule reliant on sycophancy and corruption.

The Kremlin had dismissed Navalny’s claims of corruption and Putin’s personal wealth. Navalny’s movement is outlawed and most of his senior allies fled Russia and now live in Europe.

Russian officials cast Navalny as an extremist who was a puppet of the U.S. CIA intelligence agency which they say is intent on trying to sow the seeds of revolution to weaken Russian and make it a client state of the West.

Navalny participated in Russian nationalist marches in the 2000s. Calls for restrictions on immigration and criticism over what some viewed as his overly nationalist views prompted his expulsion from the liberal Yabloko opposition party in 2007.

When demonstrations against Putin flared in December 2011, after an election tainted by fraud accusations, he was one of the first protest leaders arrested.

In an interview in Moscow in 2011, Navalny was asked by Reuters if he was afraid of challenging Putin’s system.

“That’s the difference between me and you: you are afraid and I am not afraid,” he said. “I realize there is danger, but why should I be afraid?”

Navalny’s last post on Telegram was a Valentine’s Day message to his wife Yulia below a picture of them together.

“Baby, you and I have everything like in the song: cities between us, airfield take-off lights, blue blizzards and thousands of kilometres. But I feel that you are there every second, and I love you more and more,” Navalny said.