The Save Oleg Sentsov group said on Twitter late on May 22 that the campaign, dubbed #SaveOlegSentsov, is being organized for June 1-2, ahead of this summer's World Cup soccer competition in Russia, RFE/RL reported.

Sentsov, who is a native of Crimea, is currently serving a 20-year prison term after being convicted on terrorism charges that he and human rights groups say were politically motivated.

"In different cities around the world, we will show a red card to [Russian President Vladimir] Putin's regime, which illegally holds people behind bars," the group said.

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Separately, a demonstration is being planned for New York City's Time Square on May 26 to protest against Sentsov's detention, according to organizers who contacted RFE/RL.

Sentsov, who is being held in the far-northern Yamalo-Nenets Autonomous Region, said he began a hunger strike on May 14.

He is demanding the release of 64 Ukrainian citizens that he considers to be political prisoners in Russia.

Sentsov, 41, was arrested in May 2014 on suspicion of planning fire-bombings of pro-Russian organizations in Crimea. A Russian court convicted him on multiple terrorism charges in August 2014.

Sentsov has denied all charges against him, saying that a "trial by occupiers cannot be fair by definition."

The prominent Russian human rights group Memorial has recognized Sentsov as a political prisoner, and international rights organizations have called for his release.

Volodymyr Balukh, a pro-Kyiv activist imprisoned by Russian authorities in Crimea in another politically charged case, has been on a hunger strike for nearly two months.