 Well-known critic of Vladimir Putin, Alexei Navalny, has died according to the Russian prison service. In a statement on its website, the federal penitentiary service of the Yamilonenets autonomous district said Navalny felt unwell after a walk on Friday. He almost immediately lost consciousness, according to the statement. The prison service said medical staff were called but they were unable to resuscitate the public figure. His death leaves Russia's opposition, hampered by years of harassment and prosecution, without a clear figurehead. All of Putin's most high-profile critics are now either dead, jailed or in exile. But Navalny was, undoubtedly, the biggest thorn in the Kremlin's side. For over a decade, he led nationwide protests against the authorities, ran for office to challenge members of the Russian establishment and set up a network of campaign offices across the country that have since been dismantled. For a decade, Navalny was tolerated by the Kremlin. He endured a seemingly endless succession of arrests, court appearances and periods in detention but he survived. He was arrested at passport control for supposedly breaking parole and placed in pretrial detention. Born in 1976 in the tiny town of Bytyen, near Moscow, Navalny was educated as a lawyer and economist but entered politics in 2008, starting his anti-corruption fund, FBK, three years later. He was known for his oratory skills as well as his use of the online space to promote the results of his investigations and spread his ideal of what he called the, wonderful Russia of tomorrow. His digital savvy made him particularly popular among Russia's more democratically-minded teens and youth. Navalny rose to prominence as Russia's most outspoken Kremlin critic after leading a series of anti-corruption investigations into top members of the Russian elite.