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

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 1,900 kilometers (1,200 miles) northeast of Moscow, as yet another attempt to force Navalny into silence.

The remote region is notorious for long and severe winters. Kharp is about 100 kilometers (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, 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.