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. He had "almost immediately lost consciousness", it said in a statement. The causes of his death were being established, Tass news agency reported.
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