Russia's most significant opposition leader for the past decade, Alexei Navalny, has died in prison inside the Arctic Circle, the prison service said.

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.

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

The causes of his death were being established, Tass news agency reported.

This breaking news story is being updated and more details will be published shortly.

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