Alexei Navalny, a critic of Russian President Vladimir Putin and a leading figure in the country's opposition movement, passed away after reportedly losing consciousness at the "Polar Wolf" penal colony, located 1,200 miles northeast of Moscow.
Prison authorities stated that Navalny felt unwell following a walk on Friday, and despite efforts by medical staff to revive him, they were unsuccessful.
Mr Navalny, who was serving a 19-year sentence on charges of extremism, was moved in December from his former prison in the Vladimir region of central Russia.