FILE PHOTO: Russian opposition leader Alexei Navalny participates in a hearing against the Ministry of Justice, in Moscow

Russian opposition leader Alexei Navalny appears on a screen via a video link from the IK-3 penal colony in Kharp in the Yamal-Nenets region during a hearing in Supreme Court, in Moscow on Jan. 11.

Alexei Navalny, the Russian lawyer and anti-corruption activist who became the most potent voice in opposition to President Vladimir Putin, a calling that landed him in a maximum-security prison camp, has died, according to the prison service. He was 47.

Navalny fell sick during a walk and medical staff were unable to revive him, the prison authorities said Friday. An investigation is under way, according to the statement. No cause of death was given.