Alexei Navalny, the fiercest foe of Russian President Vladimir Putin who crusaded against official corruption and staged massive anti-Kremlin protests, died in prison on Friday, Russia’s prison agency said.

Navalny was 47.

The federal prison service said in a statement that Navalny felt unwell after a walk on Friday and lost consciousness.

An ambulance arrived to try to rehabilitate him, but he died.

Kremlin spokesman Dmitry Peskov says Putin was informed of Navalny’s death and the prison service was looking into the death in line with standard procedures.

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Navalny’s spokeswoman, Kira Yarmysh said on X, the platform formerly known as Twitter, that the politician’s team had no confirmation of his death so far and that his lawyer was traveling to the town where he was held.

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 to to a “special regime” penal colony – the highest security level of prisons in Russia – above the Artic Circle.

His allies decried the transfer to a colony in the town of Kharp, in the Yamalo-Nenets region about 1,200 miles north-east of Moscow, as yet another attempt to force Navalny into silence.

The remote region is notorious for long and severe winters.

Kharp is about 60 miles from Vorkuta, whose coal mines were part of the Soviet gulag prison-camp system.

Navalny had been behind bars since January 2021, when he returned to Moscow after recuperating in Germany from nerve agent poisoning that he blamed on the Kremlin.

Before his arrest, he campaigned against official corruption, organised major anti-Kremlin protests and ran for public office.

He had since received three prison sentences, all of which he rejected as politically motivated.

He was 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. A higher court later gave him a suspended sentence.

The National: Alexei Navalny

The day before the sentence, 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 add a tinge of legitimacy to the mayoral election.

Navalny finished second, an impressive performance against the incumbent who had the backing of Mr Putin’s political machine and was popular for improving the capital’s infrastructure and aesthetics.

Navalny’s popularity increased after the leading charismatic politician, Boris Nemtsov, was shot and killed in 2015 on a bridge near the Kremlin.

Whenever Putin spoke about 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.