VLADIMIR Putin's top critic Alexei Navalny has died in a hellhole Siberia prison as Kremlin challengers claim he was murdered.

Prison officials today said the jailed Russian opposition leader, 47, collapsed inside a brutal Polar Wolf jail where he was being kept and could not be resuscitated.

Navalny was one of Putin's greatest enemies and his leading domestic critic in Russia - bravely challenging the tyrant's brutal war in Ukraine.

He had suffered serious health problems in jail, and was - according to prison officials - out for a walk in the prison compound when he mysteriously collapsed.