Russian opposition leader Alexei Navalny, a prominent critic of President Vladimir Putin known for his anti-corruption campaigns and protests against the Kremlin, has died in prison at the age of 47, according to Russia's prison agency.

The Federal Penitentiary Service stated that Navalny fell ill after a walk on Friday and subsequently lost consciousness.

Despite efforts by an ambulance crew to revive him, Navalny passed away.

Kremlin spokesman Dmitry Peskov confirmed that President Putin was informed of Navalny's death and that the prison service is investigating the incident according to standard procedures.

Navalny's spokesperson, 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, serving a 19-year sentence on extremism charges, was moved in December from his former prison in the Vladimir region of central Russia to a "special regime" penal colony above the Arctic Circle.

His transfer to the colony in Kharp, in the Yamalo-Nenets region about 1,900 kilometers (1,200 miles) northeast of Moscow, was criticized by his allies as an attempt to silence him.

Navalny had been in custody since January 2021, when he returned to Moscow after recovering in Germany from nerve agent poisoning, which he blamed on the Kremlin.

He campaigned against official corruption, organized major anti-Kremlin protests, and ran for public office.

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

Navalny, born in Butyn, about 40 kilometers outside Moscow, 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 political and business circles, including buying a stake in Russian oil and gas companies to become an activist shareholder and push for transparency.

Navalny's work on corruption resonated with Russians' sense of being cheated, giving him more influence than abstract concerns about democratic ideals and human rights.

He was convicted in 2013 of embezzlement, a prosecution he called politically motivated, and sentenced to five years in prison. The prosecutor's office later surprisingly demanded his release pending appeal, and a higher court gave him a suspended sentence.

Navalny's release came a day before he registered as a candidate for Moscow mayor. He finished second in the mayoral election against the incumbent, backed by Putin's political machine.

His popularity increased after the 2015 killing of Boris Nemtsov, a leading opposition politician, near the Kremlin.

Putin never mentioned Navalny by name, referring to him as "that person" or similar wording, apparently trying to diminish his importance.