Russian opposition leader Alexei Navalny has died aged 47 in a penal colony, the country’s prison service said on Friday.
The prison authorities claimed in a statement that Mr Navalny felt unwell after a walk on Friday and lost consciousness.
An ambulance arrived to try to rehabilitate him, but he died, it further alleged.
There was no immediate confirmation of Mr Navalny's death from his team.
The Kremlin said it had no information on the cause of his death.
Mr Navalny 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 claimed.
By far Russia's most famous opposition leader, Mr Navalny rose to prominence more than a decade ago by lampooning the elite class round President Vladimir Putin and voicing allegations of corruption on a vast scale.
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.
Navalny, the prison service said, had lost consciousness almost immediately.
"The medical staff of the institution arrived immediately, and an ambulance team was called," the prison service alleged.
"All necessary resuscitation measures were carried out, which did not yield positive results. Doctors of the ambulance stated the death of the convict."
"The causes of death are being established."
Russian prisons are notorious for their violence and ill-treatment of inmates.
Putin has been told about Mr Navalny's death, Kremlin spokesman Dmitry Peskov said.
Mr Navalny earned admiration from Russia's disparate opposition for voluntarily returning to Russia in 2021 from Germany, where he had been treated for what Western laboratory tests showed was an attempt to poison him with a nerve agent.
Mr Navalny said at the time that he was poisoned in Siberia in August 2020. The Kremlin denied trying to kill him and said there was no evidence he was poisoned with a nerve agent.
But the Russian authorities have been accused by Britain of lying about the 2018 Salisbury poisonings using the military grade nerve agent Novichok to target former double-agent Sergei Skripal.
