Jailed Russian opposition leader Alexei Navalny is dead, the prison service of the Yamalo-Nenets region where he had been serving his sentence said on Friday.

In a statement published on its website, the Federal Penitentiary Service of the Yamalo-Nenets Autonomous District said that Navalny “felt unwell” after a walk on Friday, and “almost immediately lost consciousness.”

It said that medical staff had been called, but that they were unable to resuscitate Navalny. It said the reason of death was being established.