(Associated Press) – Alexei Navalny, who crusaded against official corruption and staged massive anti-Kremlin protests as President Vladimir Putin’s fiercest foe, has died in the Arctic penal colony where he was serving a 19-year sentence.

That’s according to Russia’s prison agency.

He was 47.

The stunning news comes less than a month before an election that will give Putin another six years in power.

It brought renewed criticism of the Kremlin leader who has cracked down on all opposition at home.

The Federal Penitentiary Service said in a statement that Navalny felt unwell after a walk on Friday and lost consciousness.

An ambulance arrived but paramedics failed to revive him.