Anti-Kremlin activist Ildar Dadin, who was imprisoned in Russia for protesting against Vladimir Putin, was killed on the front line in Ukraine where he was fighting alongside Kyiv forces, his relatives and Russian media said Sunday.

“It is with deep regret that I must inform you that Ildar Dadin – call sign Gandhi – died yesterday in combat in the Kharkiv region in northeast Ukraine, his friend and former Russian MP Ilya Ponomaryov, who is living in exile, announced on Facebook.

He hailed Dadin as a “fearless and determined fighter” who had gone “to the front to fight Putinism”.

Dadin, 42, had fought alongside Ukrainian armed forces in a “Siberian battalion”, according to Ponomaryov, and then in the “Freedom Legion of Russia”, a group claiming to be made up of Russians and responsible for several incursions into the country.

His death was confirmed on Telegram by Russian journalist Ksenia Larina and several other independent Russian media outlets.

Dadin had been sentenced in 2015 to 2,5 years in prison in Russia for organizing protests alone against authorities.

Detained in a particularly harsh prison camp, he denounced the torture and humiliation inflicted on him by his prison guards in a letter published by independent media Meduza in 2016.

Dadin was the first Russian citizen to be convicted under a 2014 law clamping down on protests in the country.