One of Russia's most prominent opposition politicians and frequent critic of President Vladimir Putin, Alexei Navalny, has died in prison, Reuters reported, citing a news release from the state's prison service.

Lost consciousness

Navalny, who was incarcerated in a Russian penal colony in the Arctic, was said to feel unwell after a walk on Friday (Feb. 16) and "almost immediately lost consciousness".

Medical staff were called, but they were unable to resuscitate him, the prison service said.

Navalny's cause of death is currently being established.

"Putin's harshest critic"

Having gained prominence in the past decade, Navalny was considered one of Putin's harshest critics.

He handed a 19-year jail term in 2023 for supporting "extremism", New York Times (NYT) reported, on charges his supporters claimed were politically-motivated.

At the time, Navalny was already serving a 9-year-sentence on conviction for fraud.

Navalny was then moved to one of the toughest prisons in Russia in late 2023, BBC reported.

When the sentence was pronounced, Navalny said that the number of years didn't matter, according to the NYT.

"I understand very well that, like many political prisoners, I am serving a life sentence. Where life is measured by the duration of my life or the life span of this regime."

Election in March

Russia will be holding its presidential election from Mar. 15 to 17 this year and inaugurate the winner in May 2024, Reuters reported.

Putin had expressed his intention to run in the 2024 presidential election.

Should he win the election, Putin will remain in power until at least 2030.

Top image Алексей Навальный/Facebook.