Kremlin critic and activist of the Left Front opposition group Konstantin Kosyakin died in Moscow on Saturday, the group said. He was 66.
Left Front leader Sergei Udaltsov said in a tweet that Kosyakin died of cancer and that his death is "a great loss" to Russia's protest movement.
Kosyakin was among the leaders of the radical Communist group known for its harsh criticism of President Vladimir Putin's policies. The group's activists have been under pressure in recent months amid a widening Kremlin crackdown on opposition and critics.
Kosyakin was among the first organizers of opposition rallies on the 31st of each month with that many days, a reference to Article 31 of the Russian constitution that guarantees freedom of assembly, the group said. The small rallies have been dispersed by police and its participants, including Kosyakin, detained.
Born in 1947, Kosyakin was an activist with the Communist Party of the Russian Federation until 2004, according to his online biography on the Lenta.ru news website. Since then, he joined several Communist and socialist groups such as the Avant-Garde of Red Youth that later merged to form the Left Front, the biography said.