He was 47.

He was serving a 19-year term.

Alexey Navalny felt unwell in prison after a walk and lost consciousness. He received all necessary resuscitation measures, but they did not yield positive results, the Federal Ptnitentiary Service stated as quoted by Russia's Sputnik news agency.

Authorities are trying to figure out the cause of his death.
 
Commenting on the development, Kremlin spokesman Dmitry Peskov was quoted as saying by Sputnik it is up to medics to find out reasons behind Navalny's death.

“As far as we know, now, in accordance with all current rules, the Federal Penitentiary Service is engaged in all inspections, clarifications, and so on. That is, no instructions are required anymore, because there is a certain set of rules in this regard that the service is now guided by,” Peskov told reporters, adding that the news about Navalny’s death was “reported from Moscow.”

Speaking about the investigation into the death of the Russian leader, authority said in a statement as quoted by Sputnik: "The investigation office of the Russian Investigative Committee in the Yamalo-Nenets Autonomous Area has launched a procedural investigation into the death of Navalny A.A. ... In accordance with the procedure established by law, a set of investigative and operational measures is being carried out aimed at establishing all the circumstances of the incident.

The prominent opposition leader had garnered a massive follower base for his criticisms of the corruption in Putin's administration.

According to reports, he returned to Russia in 2021 from Germany where he underwent treatment after being allegedly poisoned.

Upon his return, Navalnvy was swiftly arrested on charges he dismissed as politically motivated, reported CNN.

Reaction from Navalny's team

Navalny's close aide Leonid Volkov wrote on X: "Russian authorities publish a confession that they killed Alexey Navalny in prison."

"We do not have any way to confirm it or to prove this isn’t true. Navalny’s lawyer is on the way to Harp," he said.

Navalny’s spokesperson, Kira Yarmysh, posted on X: "The Federal Penitentiary Service of Russia in the Yamalo-Nenets Autonomous Okrug is spreading the news of Alexey Navalny's death in IK-3. We have no confirmation of this yet. Alexey's lawyer is currently on his way to Kharp. As soon as we have some information, we will report on it."