The Russian prison service has announced the death of Alexei Navalny, the country’s most prominent opposition figure of the past decade, while he was incarcerated in a jail within the Arctic Circle.

Navalny, a staunch critic of President Vladimir Putin, was serving a 19-year sentence on charges of extremism widely believed to be politically motivated. He had been transferred to one of Russia’s harshest penal colonies late last year.

His wife, Yulia, has appealed to the international community for assistance in holding the Russian regime accountable.

Ivan Zhdanov, chief of the Anti-Corruption Foundation and one of Navalny’s closest allies, expressed strong suspicions that Navalny had been deliberately killed, describing it as a probable murder.

According to the prison service in Russia’s Yamalo-Nenets district, where Navalny was held, he became unwell after a walk on Friday and quickly lost consciousness. Despite immediate efforts by emergency medical personnel to revive him, they were unsuccessful.

A statement from the prison service reported that Navalny was pronounced dead by the emergency doctors, and an investigation into the cause of death is ongoing.

Following the news, residents of Moscow paid tribute to Navalny by laying flowers at a monument dedicated to victims of political repression, situated opposite the Lubyanka building, formerly the headquarters of Russia’s notorious KGB spy service.