Alexei Navalny was 'brutally murdered by the Kremlin,' Latvia's president has said, after Russia reported today the jailed opposition leader had 'collapsed and died'.

Russian news outlets announced Navalny's death - citing the Siberian prison service where he was serving his sentence - sparking shock and anger around the globe, with world leaders quickly pointing the finger at Russian president Vladimir Putin.

As Putin's fiercest foe, Navalny crusaded against official corruption and staged massive anti-Kremlin protests - drawing Moscow's merciless retribution.

He once survived an assassination attempt involving the Novichok nerve agent, but returned to his homeland upon his recovery, despite knowing he would be arrested.

Ever since, he has been serving time on extremism charges after being sentenced to 19 years behind bars, and in December was moved from a prison in central Russia to a 'special regime' penal colony known as 'Polar Wolf' above the Arctic Circle.

The 47-year-old was last seen via video link during a court hearing on Thursday. 

Dressed in black prison uniform, he appeared to be in good spirits - his trademark humour back on show. 'Your Honour, I will send you my personal account number so that you can use your huge salary as a federal judge to 'warm up' my personal account, because I am running out of money,' he said.

Navalny's mother Lyudmila said she had seen her son in the prison colony on Monday. At the time, she said: 'He was alive, healthy, cheerful.'

But the Federal Prison Service said in a statement today that Navalny felt unwell after a walk and lost consciousness. An ambulance arrived to try to save him, to no avail.

Speaking at the Munich Security Conference just hours after the news of her husband's death broke, Yulia Navalnaya said she was sceptical because it had come from Russian government sources.