MOSCOW, Russia (AFP)— Russian opposition leader Alexei Navalny died Friday in the Arctic prison colony where he was serving a 19-year-term, Russia’s federal penitentiary service said.

Western governments immediately attacked the Kremlin over the death of the most outspoken critic of President Vladimir Putin.

Navalny lost consciousness after a walk and could not be revived by medics, the prison service said.

“Navalny felt bad after a walk, almost immediately losing consciousness. Medical staff arrived immediately and an ambulance team was called,” it said.

“Resuscitation measures were carried out which did not yield positive results. Paramedics confirmed the death of the convict. The causes of death are being established.”

The 47-year-old was Russia’s most prominent opposition leader and won a huge following with his criticism of corruption in Vladimir Putin’s Russia.

Russia’s Investigative Committee said it had opened an investigation into the death.

Navalny’s press secretary Kira Yarmysh said his team had not been informed of his death.

“Alexei’s lawyer is now flying to Kharp,” where his prison colony is, she said in a post on social media.

Citing his spokesman, Russian news agencies reported that Putin had been informed of Navalny’s death.

Western governments and Russian opposition figures on Friday said the Kremlin was responsible for his death.

Latvia’s president said he had been “brutally murdered by the Kremlin”.

“The Russian government bears a heavy responsibility,” Norway’s Foreign Minister Espen Barth Eide wrote on X, formerly Twitter.

France’s foreign minister said Navalny had paid with his life for resisting oppression.

Navalny’s exposes, posted on his YouTube channel racked up millions of views and brought tens of thousands of Russians to the streets, despite Russia’s harsh anti-protests laws.

He was jailed in early 2021 after returning to Russia from Germany, where he was recuperating from a near-fatal poisoning attack with Novichok, a Soviet-era nerve agent.

In a string of cases he was sentenced to 19 years in prison on charges widely condemned by independent rights groups and in the West as retribution for his opposition to the Kremlin.

His return to Russia despite facing jail put him on a collision course with Putin, after Navalny blamed the poisoning attack in Siberia on the Kremlin.

“I’m not afraid and I call on you not to be afraid,” he said in an appeal to supporters as he landed in Moscow, moments before being detained on charges linked to an old fraud conviction.

His 2021 arrest spurred some of the largest demonstrations Russia had seen in decades, and thousands were detained at rallies nationwide calling for his release.

In prison, Navalny’s team said he had been harassed and repeatedly moved to a punitive solitary confinement cell.

He said guards had subjected him and other inmates to “torture by Putin”, making them listen to the president’s speeches.

From behind bars he was a staunch opponent of Moscow’s full-scale military offensive against Ukraine.

The Kremlin moved to dismantle his organisation, locking up his allies and sending dozens of others into exile.

Late last year he was moved to a remove Arctic prison colony in Russia’s Yamalo-Nenets region in northern Siberia.

The last post on Navalny’s Telegram channel, which he managed through his lawyers and team in exile, was a tribute to his wife, Yulia Navalnaya, posted on Valentine’s Day.