Alexei Navalny, Russia's most famous opposition leader, died on Friday after collapsing and losing consciousness at the penal colony north of the Arctic Circle where he was serving a long jail term, the Russian prison service said.
Navalny, 47, rose to prominence more than a decade ago by lampooning President Vladimir Putin and the Russian elite whom he accused of vast corruption, avarice and opulence.
The Federal Penitentiary Service of the Yamalo-Nenets Autonomous District said in a statement that Navalny "felt unwell" after a walk at the IK-3 penal colony in Kharp, about 1,900 km (1,200 miles) north east of Moscow.
He lost consciousness almost immediately, it said.
"All necessary resuscitation measures were carried out, which did not yield positive results. Doctors of the ambulance stated the death of the convict," the prison service said, adding that causes of death were being established.
Kremlin spokesman Dmitry Peskov said on Friday that Russia's penitentiary service was making all checks regarding the death of jailed opposition leader Navalny, but that he had no information about the matter.
Russian president Vladimir Putin has been told about Navalny's death, the spokesman added. Putin, who is running for re-election in a month, was shown on a television clip visiting a factory in the Urals.
