Alexei Navalny, Russia's top opposition leader and President Vladimir Putin's fiercest foe, died in prison on Friday, a statement from the Federal Penitentiary Service said. Navalny, who was serving a 19-year sentence on charges of extremism, felt unwell after a walk and collapsed, it said. Navalny was moved in December from his former prison in central Russia to to a “special regime” penal colony — the highest security level of prisons in Russia — above the Arctic Circle.