Russia's most significant opposition leader for the past decade, Alexei Navalny, has died in an Arctic Circle jail, the prison service said.
Seen as President Vladimir Putin's most vociferous critic, Navalny was serving a 19-year jail term on charges widely considered politically motivated.
He was moved to one of Russia's toughest penal colonies late last year.
The prison service in the Yamalo-Nenets district said Navalny 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."
Navalny, 47, was last seen only a day ago, looking well and laughing during a court hearing via video link.
His lawyer Leonid Solovyov told Russian media he would not be commenting yet, although Navalny's close aide Leonid Volkov wrote on X: "Russian authorities publish a confession that they killed Alexei Navalny in prison. We do not have any way to confirm it or to prove this isn't true."
BBC - overview of Navalny's life