Alexei Navalny, Russian President Vladimir Putin's most formidable domestic opponent, fell unconscious and died on Friday after a walk at the "Polar Wolf" Arctic penal colony where he was serving a three-decade sentence, authorities said.
The death of Navalny, a 47-year-old former lawyer, robs the disparate Russian opposition of its most prominent leader as Putin prepares for an election which will keep the former KGB spy in power until at least 2030.
U.S. President Joe Biden said he was outraged and joined other Western leaders in criticising Russia over Navalny's death, blaming it on "something that Putin and his thugs did."
Navalny rose to prominence more than a decade ago by documenting and speaking publicly about what he said was the vast corruption and opulence among the "crooks and thieves" running Putin's Russia.
There are no other opposition leaders in Russia of Navalny's stature. For some young urban Russians, Navalny had offered hope of an alternative future to Putin, who has served as Russia's paramount leader longer than anyone since Josef Stalin.
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) northeast of Moscow.
He lost consciousness almost immediately and died shortly afterwards, it said, adding that resuscitation attempts failed.
Navalny's spokeswoman, Kira Yarmysh, said there was "almost no hope" that he was alive.
Navalny's wife, Yulia, told the Munich Security Conference she could not be sure her husband was dead because "Putin and his government... lie incessantly".
"But if this is true, I want Putin, his entire entourage, Putin’s friends, his government to know that they will bear responsibility for what they did to our country, to my family, to my husband," she said.
The Kremlin said Putin had been informed of Navalny's death. The 71-year-old former KGB spy was shown meeting workers at a factory in Chelyabinsk, in the Ural mountains. He said nothing in public about Navalny.
Western leaders paid tribute to Navalny's courage as a fighter for freedom. Some, without citing evidence, accused the Kremlin of murder and said Putin should be held accountable.
Kremlin spokesman Dmitry Peskov said the reaction of Western leaders to the death was unacceptable and "absolutely rabid".
"We don't know exactly what happened, but there is no doubt that the death of Navalny was a consequence of something that Putin and his thugs did," Biden said at the White House.
"Russian authorities are going to tell their own story," Biden said. "But make no mistake. Make no mistake: Putin is responsible for Navalny's death."
MURDER ACCUSATION
Navalny's movement is outlawed and most of his senior allies now live in exile in Europe.
"If this (news of Navalny's death) is true, then it's not 'Navalny died', but 'Putin killed Navalny'," Leonid Volkov, Navalny's chief of staff, said. The Kremlin did not respond to the accusation.
Yarmysh, Navalny's spokeswoman, said his movement and aims would live on.
"We are convinced that we will be victorious in the end," she said. "Russia is our country, it belongs to us and we need to return it us."
Navalny's lawyer was on his way to the tough penal colony where Navalny was serving sentences that would have kept him in prison beyond the age of 70.
Navalny earned admiration from Russia's 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.
Grigory Yavlinsky, a veteran liberal, said Navalny's death showed the need for reform and echoed many opposition figures in saying he feared for the health of other activists in jail.
In Moscow, at a memorial to the victims of Soviet political repression in the shadow of the former KGB headquarters, some people laid roses and carnations. Police looked on.
One note read: "Alexei Navalny - we remember you."
Russian prosecutors warned people not to take part in any mass meetings in Moscow. Supporters arranged meetings to honour Navalny in London, Paris, Oslo, Rome, Brussels, Berlin, Geneva, Prague, Yerevan, Tbilisi and Vilnius.
RUSSIA'S FUTURE
Navalny had forecast Russia could face political turmoil because he said Putin built a brittle system of personal rule reliant on corruption. The Kremlin dismissed his accusations about vast corruption and about Putin's personal wealth.
Russian officials cast Navalny as an extremist who was a puppet of the CIA, which they say is intent on sowing chaos and turning Russia into a client state of the West.
A day before his death, Navalny peered through a barred window, laughing and cracking jokes about his depleting funds and the judge's salary.
"Your Honour, I will send you my personal account number so that you can use your huge salary as a federal judge to 'warm up' my personal account, because I am running out of money," he said via video link.
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?"
