Russian opposition figure Alexei Navalny has died, prison officials in the Yamal-Nenets region said on Friday.
The Federal Prison Service said Navalny felt unwell after a walk on Friday and lost consciousness. Although an ambulance arrived and "all necessary resuscitation measures" were used, he died.
The causes of death are being established, a statement from the prison service said.
Navalny's spokeswoman Kira Yarmysh said she has have no confirmation of Navalny's death at the prison in the town of Kharp.
"Alexei's lawyer is now flying to Kharp. As soon as we have some information, we will report it," she said.
In late December, Yarmysh said Navalny was at the "Polar Wolf" colony, officially known as the IK-3 penal colony, about 1,900km northeast of Moscow.
Navalny himself later tweeted that he was he was "fine" despite a "pretty exhausting" transfer that took 20 days.
The 47-year-old has made a name for himself by publicly calling out graft at the top of the Kremlin hierarchy. He is the founder of the FBK, and was one of the leaders of the anti-government protests in 2011-2013, which culminated in him running for the mayor of Moscow in 2013. He has called Russian President Vladimir Putin a "madman" and said the ruling party in Russia was full of "crooks and thieves."
Over the years, Navalny has faced physical attacks, repeated arrests, investigations, and criminal proceedings, which he claims to be politically motivated. Most notably, he survived an assassination attempt in 2020 which involved the nerve agent Novichok. After the Novichok poisoning, Russian officials agreed to fly comatose Navalny out to Germany for treatment, with Navalny waking up and eventually recovering in Berlin's Charite hospital. However, with Navalny staying in Germany for several months after his release from the hospital, Russian authorities warned he was in violation of probation terms of an earlier court case and threatened to arrest him upon return.
Despite the warning, Navalny chose to return to Moscow in January 2021. Russian authorities detained him at the airport, and the politician has been behind bars ever since. While serving the sentence for the parole violation, he was sentenced to nine years for embezzlement and contempt of court. Another court ruling in August this year saw his prison term prolonged to 19 years over additional charges including extremism.
German Health Minister Karl Lauterbach was among the first to pay tribute to Navalny.
"Today we no longer talk about heroes, but for me Navalny was a hero," said Lauterbach. "Through his resistance, he made it clear to the world early on that Putin is a ruthless criminal in office. His warning should have been heeded much earlier rest in peace."
