Russia’s prison agency announced Friday that prominent Vladimir Putin critic and opposition leader Alexei Navalny has died at the age of 47.

The Federal Prison Service said in a statement that Navalny felt unwell after a walk on Friday and lost consciousness, according to The Associated Press. An ambulance arrived to try to rehabilitate him, but he died, the statement added.

Navalny was being held at the IK-3 penal colony, also known as "Polar Wolf," in Kharp in northern Russia, which is considered one of the country's toughest prisons.

Navalny has previously organized anti-government demonstrations and has run for office to advocate for reforms against what he claims is corruption in Russia. He was the victim of an alleged assassination attempt in 2020, when he suffered poisoning from a suspected Novichok nerve agent. 

He remained in a coma for several weeks while doctors in Germany fought to keep him alive. He accused Putin of being responsible for his poisoning.

Navalny then returned to Russia in 2021, when authorities immediately arrested him and later sentenced him to 19 years in prison on extremism charges. His team has repeatedly raised concerns about his treatment following his return and Navalny has said the charges were politically motivated.

There was no immediate confirmation of Navalny’s death from his team.

This is a developing story. Please check back for updates.

Fox News' Michael Dorgan and The Associated Press contributed to this report.