Navalny's lawyer says that "everything was fine with Navalny" when he visited him two days ago.
He had been in jail since January 2021. At the time, Biden warned of "devastating" consequences for Russia if Navalny dies in prison.
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The authorities say Mr Navalny felt unwell following a walk and lost consciousness at the "Polar Wolf" penal colony inside the Arctic Circle - but while the cause of death remains unknown, Western leaders have made clear they hold the Kremlin ultimately responsible.
Russian opposition leader Alexei Navalny - a vocal critic of President Vladimir Putin - has died, according to the country's prison service.
The jailed dissident, who had campaigned against official corruption and led major anti-Kremlin protests, was 47.
Mr Putin, who is running for re-election in a month, has been informed of his death, according to the state news agency TASS.
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Navalny's wife holds Putin 'personally liable'
While the cause of death remains unknown, Western leaders have made clear they hold the Russian regime ultimately responsible.
The prison authorities said Mr Navalny felt unwell following a walk on Friday and lost consciousness at the "Polar Wolf" penal colony about 1,200 miles (1,900 km) northeast of Moscow, and within the Arctic Circle.
Efforts by medical staff to revive him failed, according to the service.
Mr Navalny, who was serving a 19-year sentence on charges of extremism, was moved in December from his former prison in the Vladimir region of central Russia.
His allies criticised the transfer to the maximum security facility in the town of Kharp as yet another attempt to silence Mr Navalny.
The remote region is notorious for long and severe winters.
Mr Navalny had been behind bars since January 2021, when he returned to Moscow after recovering in Germany from nerve agent poisoning, which he blamed on the Kremlin.
Speaking at a security conference in Munich, his wife Yulia Navalnaya, said: "For many years we cannot trust Putin and the Putin government. They always lie.
"But if this is true, I want Putin and everyone around him to know that they will be held accountable for everything they did to our country, to my family. And this day will come very soon.
"I want to call on the international community and all people to unite and defeat this evil."
His mother Lyudmila Navalnaya is reported to have written on Facebook: "I don't want to hear any condolences.
"We saw him in prison on the (February) 12, in a meeting.
"He was alive, healthy and happy."
Nobel Peace Prize winner and campaigning journalist Dmitry Muratov, who is editor-in-chief of Russia's most famous independent newspaper, Novaya Gazeta, said: "My sincere belief is that it was the conditions of detention that led to Navalny's death... His sentence was supplemented by murder."
Ukrainian leader Volodymyr Zelenskyy said: "It is obvious that he was killed by Putin."
US president Joe Biden said: "I am both not surprised, and outraged, by the news."
"Make no mistake, Putin is responsible for Navalny's death," he added, saying it was "yet more proof of Putin's brutality".
US Secretary of State Antony Blinken said: "His death in a Russian prison and the fixation and fear of one man only underscores the weakness and rot at the heart of the system that Putin has built.
"Russia is responsible for this."
