London, Feb 16 (IANS): Russia's most significant opposition leader for the past decade, Alexei Navalny, has died in prison inside the Arctic Circle, the prison service said, the media reported.

Seen as President Vladimir Putin's most vociferous critic, Navalny was serving a 19-year jail term for offences widely considered politically motivated.

He was moved to an Arctic penal colony, considered one of the toughest jails, late last year, BBC reported.

The prison service in the Yamalo-Nenets district said he had "felt unwell" after a walk on Friday.

He had "almost immediately lost consciousness", it said in a statement, adding that an emergency medical team had immediately been called and tried to resuscitate him but without success.

"The emergency doctors declared the prisoner dead. Cause of death is being established," BBC reported.

Navalny's lawyer Leonid Solovyov told Russian media he would not be commenting yet.

In early December he had disappeared from a prison in the Vladimir region, where he was serving a 30-year sentence on extremism and fraud charges that he had called political retribution for leading the anti-Kremlin opposition of the 2010s. He did not expect to be released during Putin's lifetime, The Guardian reported.