Kharp, Russia - Russian opposition leader Alexei Navalny died Friday at the Arctic prison colony where he was serving a 19-year-term.

Navalny lost consciousness after going for 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."

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Russia's Investigative Committee said it had opened an investigation into his 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 spokesperson, Russian news agencies reported that President Vladimir Putin had been informed of Navalny's death.

Navalny (47), Russia's most prominent opposition leader, won a huge following with his criticism of corruption in Vladimir Putin's Russia.

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

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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 as retribution for his opposition to the Kremlin.

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.