Updated 4.35pm 

The Kremlin's most prominent critic Alexei Navalny died on Friday in an Arctic prison, Russian officials said, a month before an election poised to extend Vladimir Putin's hold on power.  

Navalny's death after three years in detention and a poisoning that he blamed on the Kremlin deprives Russia's opposition of its figurehead at a time of intense repression and Moscow's campaign in Ukraine.

Dissidents and Western officials blamed Putin and his government for the 47-year-old's death, which followed months of deteriorating health in harsh detention conditions.

"Alexei Navalny was tortured and tormented for three years... Murder was added to Alexei Navalny's sentence," Russian Nobel Peace Prize winner Dmitry Muratov was quoted as saying by the Novaya Gazeta newspaper.

Russian news agencies reported that medics from a hospital in Russia's Far North spent more than "half an hour" trying to resuscitate Navalny, who reportedly lost consciousness after a walk.

One of the last images of Navalny taken in January. He is seen on screen via a video link from the penal colony above the Arctic circle during a hearing of his complaint on restrictions. Photo: AFP

Kremlin spokesman Dmitry Peskov told reporters that he did not know any further details about the cause of death.

Navalny's wife, Yulia, said she held Putin personally responsible for her husband's death.

Navalny was Russia's most prominent opposition leader and won a huge following with his campaigning against corruption under Putin. 

The Russian leader -- who famously never referred to Navalny by name -- was on a visit to the Urals on Friday and made no mention of the death in his public appearance.    

'Brutally murdered'

One of Navalny's lawyers, Leonid Solovyov, told the independent Novaya Gazeta paper that the Kremlin critic was "normal" when a lawyer saw him on Wednesday.

In footage of a court hearing from his prison colony on Thursday, Navalny was seen smiling and joking as he addressed the judge by video link. State media reported he raised no health complaints during the session.

Speaking at the Munich Security Conference hours after news of her husband's death, Yulia Navalnaya called on the international community to "unite and defeat this evil, terrifying regime".

"I want Putin and all his entourage, his friends and his government to know that they will be punished for everything they have done to our country, to my family and to my husband. And that day will come very soon," she said.

Western governments and Russian opposition figures immediately blamed the Kremlin. 

US Vice President Kamala Harris said his death was "a sign of Putin's brutality," while US Secretary of State Antony Blinken said: "Russia is responsible for this."

That was echoed by European Council President Charles Michel, who said the EU held the Russia regime solely "responsible for this tragic death".

German Chancellor Olaf Scholz said Navalny had "paid for his courage with his life", while Britain's Prime Minister Rishi Sunak said his death was a "huge tragedy" for the Russian people.

The president of Latvia said he had been "brutally murdered by the Kremlin" while French Foreign Minister Stephane Sejourne said his death "reminds us of the reality of Putin's regime".

Ukraine's President Volodymyr Zelensky, battling Russian forces for the past two years, said the Kremlin critic had been "killed by Putin". 

Malta's foreign minister Ian Borg called on Russia “to provide clarity regarding the circumstances of his death".

Moscow, meanwhile, said the West was making "sweeping accusations."

Its top lawmaker Vyacheslav Volodin went further by saying that the death suited Western leaders because they were "losing" the battle in Ukraine.  

'I'm not afraid'

Navalny, who led street protests for more than a decade, became a household name through his anti-corruption campaigning.

His exposes of official corruption, posted on his YouTube channel racked up millions of views and brought tens of thousands of Russians to the streets, despite harsh anti-protest 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 knowing he would face jail brought him admiration. 

"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.

Despite being locked up, he never publicly regretted his decision to return.

"I have my country, and I have my beliefs. I don't want to give up on my country or my beliefs," he said in a January 2024 post on Instagram, delivered through his lawyers.

From behind bars he was a staunch opponent of Moscow's full-scale military offensive against Ukraine, and watched on, helplessly, as the Kremlin dismantled his organisation and locked up his allies.  

Dozens of his top supporters fled into exile and continued to campaign against the offensive on Ukraine and growing repression inside Russia.

'Don't do nothing'

Late last year, Navalny was moved to a remote Arctic prison colony in Russia's Yamalo-Nenets region in northern Siberia.

Since being jailed in 2021, he spent more than 300 days in solitary confinement, where prison authorities kept him over alleged minor infringements of prison rules.

Prison authorities sent him to solitary confinement on 27 separate occasions during his three-year incarceration.

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

In a documentary filmed before he returned to Russia, Navalny was asked what message he wanted to leave to the Russian people should he die or be killed.

"Don't give up. You mustn't, you can't give up," he said.

"All it takes for evil to triumph for good people to do nothing. Therefore, don't do nothing."