Alexei Navalny, the leader of Russian opposition against President Vladimir Putin, died Friday in prison at the age of 47, officials said.

The Russian Federal Prison Service said Navalny felt unwell after a walk and lost consciousness. An ambulance arrived and its crew tried to rehabilitate him but was unsuccessful, it added.

Navalny was serving a 19-year sentence on charges of extremism and in December was moved from a different prison to the highest-security level facility in the country near the Arctic Circle. The “special regime” penal colony prison in the town of Kharp, which is about 1,200 miles northeast of Moscow, is in a remote area known for its severe winters.

His spokesperson, Kira Yarmysh, said on X, the platform formerly known as Twitter, that her team had no confirmation of his death. Navalny’s lawyer was on his way to the prison, she said, adding that she would share more information as soon as possible.

He has been imprisoned since January, 2021 when he returned to Russia after recovering from a poisoning that he blamed on Putin. Putin has denied trying to kill Navalny with the nerve agent poison.

Navalny became a public figure for campaigning against official corruption and organizing anti-Kremlin protests while running for public office.

He had denounced his sentences and said they were politically motivated.

According to Reuters, Putin has been notified about Navalny’s death.

–The Associated Press contributed to this developing report

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