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.
Latest updates: EU 'holds Russia responsible'
While the cause of death remains unknown, world 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 Russi.
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.
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 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."
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Alexei Navalny: The one man Putin refused to mention by name
Foreign Secretary Lord Cameron said: "Navalny was an incredibly brave fighter against corruption, and he gave up everything in campaigning for what he believed in and my heart goes out to his wife and to his family.
"And we should be clear about what has happened here.
"Putin's Russia imprisoned him, trumped up charges against him, poisoned him, sent him to a arctic penal colony, and now he's tragically died.
"We should hold Putin accountable for this. And no one should be in any doubt about the dreadful nature of Putin's regime in Russia after what has just happened."
EU Council President Charles Michel said Mr Navalny had "fought for the values of freedom and democracy".
He added: "For his ideals, he made the ultimate sacrifice.
"The EU holds the Russian regime for sole responsible for this tragic death."
Mr Navalny had previously been convicted in 2013 of embezzlement on what he called a politically motivated prosecution and was sentenced to five years in prison.
But the prosecutor's office later surprisingly demanded his release pending appeal and a higher court later gave him a suspended sentence.
The day before, Navalny had registered as a candidate for Moscow mayor.
The opposition saw his release as the result of large protests in the capital of his sentence, but many observers attributed it to a desire by authorities to try and give legitimacy to the mayoral election.
Mr Navalny finished second - an impressive performance against the incumbent who had the backing of Mr Putin's political machine.
His popularity increased after the prominent politician, Boris Nemtsov, was shot and killed in 2015 on a bridge near the Kremlin.
Whenever Mr Putin spoke about Mr Navalny, he made it a point to never mention the activist by name, referring to him as "that person" or similar wording, in an apparent effort to diminish his importance.