Alexei Navalny, the fiercest foe of Russian President Vladimir Putin who crusaded against official corruption and staged anti-Kremlin protests, died in prison Friday, Russian authorities said. He was 47.

Navalny, who was serving a 19-year sentence on charges of extremism, felt unwell after a walk, according to the Federal Penitentiary Service, and lost consciousness. An ambulance arrived to try to revive him, but he died. It said the cause of death was “being established.”

Kremlin spokesman Dmitry Peskov said Putin was informed of Navalny’s death and the prison service would look into it in line with standard procedures.

Navalny’s spokeswoman Kira Yarmysh said on X, 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 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, organized major anti-Kremlin protests and ran for public office.

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

Shortly after Navalny’s death was reported, the Russian SOTA social media channel shared images of the opposition politician reportedly in court yesterday. In the footage, Navalny is seen standing up and is laughing and joking with the judge via video link.

Navalny was moved in December from a prison in central Russia to a “special regime” penal colony - the highest security level of prisons in the country - above the Arctic Circle.

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

In Putin’s Russia, political opponents often faded amid factional disputes or went into exile after imprisonment, suspected poisonings or other heavy repression. But Navalny grew consistently stronger and reached the apex of the opposition through grit, bravado and an acute understanding of how social media could circumvent the Kremlin’s suffocation of independent news outlets.

Navalny was born in Butyn, about 25 miles outside Moscow. He received a law degree from People’s Friendship University in 1998 and did a fellowship at Yale in 2010.

He gained attention by focusing on corruption in Russia’s murky mix of politicians and businesses. By concentrating on corruption, Navalny’s work had a pocketbook appeal to Russians’ widespread sense of being cheated.

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 demanded his release pending appeal. A higher court later gave him a suspended sentence.

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.

Navalny finished second.