A court in Russia on Thursday convicted a pro-war activist and critic of Vladimir Putin of justifying terrorism and sentenced him to six years in prison.

Sergei Udaltsov, the leader of the Left Front movement that opposes Putin and is affiliated with the Communist party, was arrested last year.

According to Russian independent news site Mediazona, the charges against him stem from an article Udaltsov posted online in support of another group of Russian activists accused of forming a terrorist organization. Those activists were convicted earlier this month and handed sentences ranging from 16 to 22 years in prison.

Udaltsov has rejected the charges against him as fabricated. On Thursday, he denounced the verdict as “shameful” and said he was going on a hunger strike, Mediazona reported.

According to the court ruling, the activist will be serving his sentence in a maximum security penal colony.

Udaltsov was a prominent opposition figure during the 2011-12 mass protests in Russia, triggered by reports of widespread rigging of a parliamentary election. In February 2012, he took part in a meeting that then president Dmitry Medvedev held with various opposition figures.

Russian authorities have ramped up their crackdown on dissent and free speech after the Kremlin sent troops into Ukraine, relentlessly targeting rights groups, independent media, members of civil society organizations, LGBTQ+ activists and some religious groups. Hundreds of people have been jailed and thousands of others have fled the country.

In December 2023, a Moscow court sentenced Udaltsov to 40 hours of compulsory labor for violating procedures relating to organizing a rally after he was detained on Red Square, where he tried to unfurl a flag with the image of Soviet dictator Josef Stalin, according to Russia’s state news agency Tass.

Udaltsov was previously imprisoned in 2014 and sentenced to 4.5 years on charges related to his role in organizing a 2012 demonstration against Putin that turned turbulent. He was released in 2017.