Alexei Navalny died from 'sudden death syndrome' after collapsing on Friday, Russian investigators are alleged to have told the opposition leader's mother as she visited the brutal IK-3 Polar Wolf penal colony where he was being held this morning.

Lyudmila Navalnaya was seen today travelling to the colony in northern Russia, where she was told her son died after returning from a walk at 14:17 local time on Friday. 

Navalny's allies say they were denied the opportunity to see the body, which would remain with the authorities until an investigation was complete.

Navalny's lawyer, who arrived in the town of Salekhard with Navalny's mother on Saturday, was allegedly told by the prison that the body was being held in the morgue. 

A contact at the Salekhard morgue later denied the body was there - leaving yet more question marks around the shock death of one of Putin's most fierce critics.

'It's obvious that the killers want to cover their tracks and are therefore not handing over Alexei's body, hiding it even from his mother,' his team said in a post on Telegram. 

The shock death of Putin's most fierce critic has sparked a wave of vigils and protests across Russia, prompting police to crack down with force and make hundreds of arrests since Friday.