Ildar Dadin, 34, emerged from a Siberian penal colony after some 15 months behind bars for repeatedly holding unsanctioned rallies against President Vladimir Putin's rule.
Dadin -- who was declared a prisoner of conscience by Amnesty International -- has complained of torture and abuse behind bars, and his case became a cause celebre for those who oppose the Kremlin.
"I will continue to fight against Putin's fascist regime," Dadin said in footage broadcast online by the independent Dozhd channel.
Russia's Supreme Court on Wednesday overturned Dadin's December 2015 imprisonment and ordered he be freed.
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He was originally sentenced to three years in jail, but that sentence was reduced and he currently had some six months left to run.
Dadin's case was controversial as it was the first time a protester was prosecuted under 2014 legislation that ratcheted up punishment to a maximum of five years in jail for anyone caught holding unsanctioned demonstrations more than twice in six months.
The statute -- known as Article 212.1 -- was seen as part of the Kremlin's attempt to curb dissent following mass protests against Putin in 2011-2012 and demonstrations in Kiev that ousted Ukraine's Russian-backed leader in 2014.
The decision to release Dadin came after the constitutional court criticised the harsh punishments against peaceful protestors -- sparking hope among rights groups that authorities will no longer enforce the tough sanctions.
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