A Russian court handed him a 600,000-rouble ($6,580) fine in December 2023.
Russia's Supreme Court on Wednesday rejected an appeal by sociologist and activist Boris Kagarlitsky against his five-year prison sentence on charges of "justifying terrorism", the TASS state news agency reported.
Kagarlitsky, 65, is a longtime political dissident and has spoken out repeatedly against the conflict in Ukraine in a magazine he edited and on his YouTube channel.
A petition calling for his release has been signed by intellectuals and politicians from around the world including Canadian author Naomi Klein, former British Labour Party leader Jeremy Corbyn and former Greek finance minister Yanis Varoufakis.
A former teacher at the prestigious Moscow Higher School of Social and Economic Sciences, Kagarlitsky was designated a "foreign agent" by the Russian government in 2018 and was last year added to a register of "terrorists and extremists".
A Russian court handed him a 600,000-rouble ($6,580) fine in December 2023 after finding him guilty of "publicly justifying terrorism" in a video commenting on a Ukrainian attack on the bridge linking Russia to annexed Crimea in October 2022.
The video was entitled "Explosive Congratulations for Mostik the Cat", a reference to a real cat who lived on the Crimean Bridge and whose image was used by state media to promote the idea of Crimea belonging to Russia.
Prosecutors appealed, saying the fine was too lenient, and Kagarlitsky's sentence was later increased to a five-year prison term.
In the online petition, his supporters call him a world-renowned critic of capitalism and imperialism and demand the immediate, unconditional release of Kagarlitsky and all other anti-war prisoners in Russia.
"We also call on the authorities of the Russian Federation to reverse their growing repression of dissent and respect their citizens' freedom of speech and right to protest," the petition says.
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