Russian opposition leader Alexei Navalny has died on Friday at the Arctic prison colony where he was serving a 19-year term.

Russia’s federal penitentiary service in a statement, said “Navalny felt bad after a walk, almost immediately losing consciousness. Medical staff arrived immediately and an ambulance team was called. Resuscitation measures were carried out which did not yield positive results. Paramedics confirmed the death of the convict. The causes of death are being established”.

Navalny who was permanently barred in 2018 from running in elections, has remained the most powerful political opponent of President Vladimir Putin.

The 47-year-old was being held in jail about 40 miles north of the Arctic Circle where he had been sentenced to 19 years under a “special regime”.

A former nationalist politician, Navalny, helped foment the 2011-12 protests in Russia by campaigning against election fraud and government corruption, investigating Putin’s inner circle and sharing the findings in YouTube videos that garnered hundreds of millions of views.

In 2020, Navalny fell into a coma after a suspected poisoning using novichok by Russia’s FSB security service and was evacuated to Germany for treatment.

He recovered and returned to Russia in January 2021, where he was arrested on a parole violation charge and sentenced to his first of several jail terms that would total more than 30 years behind bars.