“It’s obvious Putin killed him,” Ukrainian President Volodymyr Zelenskyy said.

“On February 16, 2024, in Penal Colony No. 3, the convict Alexei Navalny felt unwell after a walk, almost immediately losing consciousness, according to representatives of the department,” the Yamal directorate of the federal penitentiary service said in a statement.

“Medical personnel from the institution arrived promptly, and an ambulance crew was called. All necessary resuscitation measures were carried out, but unfortunately, they did not yield positive results. The emergency medical team pronounced the convict dead. The cause of death is being investigated,” the statement added.

That sudden collapse was unexpected as Navalny had appeared in good health and spirits only on Thursday. The independent Russian news outlet SOTA published a video of a court session, in which he was joked he was running out of spending money in prison.

For years, Navalny has been the most prominent opposition figure in Russia.

With the Kremlin cracking down on all forms of dissent, Navalny was seen as the only person still capable of mobilizing Russians to participate in organized protest against the Kremlin.