Moscow, Russia, February 17 (HBTV): Russia's most significant opposition leader for the past decade, Alexei Navalny, has died in an Arctic Circle jail, the prison service has said.

Navalny was President Vladimir Putin's most vociferous critic. He was serving 19 years on charges of extremism that was widely considered politically motivated.

Navalny was moved to one of Russia's toughest penal colonies the previous year.

His wife Yulia has appealed to the international community "to help punish this regime".

One of his closest allies, Anti-Corruption Foundation chief Ivan Zhdanov, said it was "highly likely" Navalny had been killed and that a "murder had actually happened".

The prison service in Russia's Yamalo-Nenets district where Navalny was being held said he had "felt unwell" after a walk on Friday.  

He had "almost immediately lost consciousness", it said in a statement. It stated that an emergency medical team was immediately called. The team tried to resuscitate Navalny but without success, reports said.

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

Hours later, a steady stream of Muscovites laid flowers at a monument to victims of political repression opposite the Lubyanka, once home to Russia's notorious KGB spy service, reports said.    

(With inputs from news agencies)