Alexei Navalny's spokeswoman confirmed on Saturday that Russia's most prominent opposition leader was dead but said it was unclear where his body had been taken as a morgue close to the Arctic prison where he died said it had not received the body.

Navalny, a 47-year-old former lawyer, fell unconscious and died on Friday after a walk at the "Polar Wolf" penal colony in Kharp, about 1,900 km (1,200 miles) northeast of Moscow, where he was serving a three-decade sentence, the prison service said.

Navalny's spokeswoman, Kira Yarmysh, said that an official death notice had been given to Navalny's mother, Lydumila. The time of death, she said, was 2:17 p.m. local time (0917 GMT) on Feb. 16, according to the notice.

"We demand that Alexei Navalny's body be given to his family immediately," Yarmysh said.

Until now, the prison service statement on Friday had been the only official word on the death of Navalny, a fierce critic of President Vladimir Putin.

Yarmysh, writing on social media platform X, said an employee of the prison had said that Navalny's body had been taken to Salekhard, the town near the prison complex, by Russian investigators, who were conducting "research".

When Navalny's lawyer and Navalny's mother, Lyudmila, arrived at the morgue in Salekhard they found it closed and were assured at Navalny's body was not there, Yarmysh said.

"It is closed, despite the assurances of the colony that it was working and that Navalny's body is there," Yarmysh said.

"The lawyer called the phone number on the door. He was told that he was the seventh person to call today. They don't have Alexei's body in the morgue."

An employee at the only morgue in Salekhard told Reuters that Navalny's body had not arrived.

"The investigative committee has a million options," the employee, who did not give their name, said, listing other cities in the region - Labytnangi, Nadym and Urengoy - where Navalny's body could have been sent to.

"In Salekhard, we have the only morgue," he said. "I have not received anyone, no bodies...no papers."

Asked whether a body could have arrived before he started his shift at 8 a.m., the employee said: "No one handed me anything. I think if such a body had been delivered to us, I would have been told immediately."

Lyudmila Navalnaya travelled on Saturday to the prison where her son had been held, accompanied by Navalny's lawyer, the Novaya Gazeta newspaper reported.

The IK-3 penal colony in Kharp is situated in the Arctic Circle, around a one-hour drive from Salekhard, the administrative capital of the Yamalo-Nenets Autonomous District.

Western leaders and officials have expressed outrage over the death of Navalny, which deprived Russian critics of Putin of their most formidable leader.

Kremlin spokesman Dmitry Peskov called the West's reaction unacceptable on Friday.

In Moscow, the death left a mood of despair and apathy. The hundreds of flowers and candles laid in the capital on Friday to honour Navalny's memory were mostly taken away overnight in black bags.

Several dozen roses and carnations remained in the softening snow on Saturday at the monument to the victims of Soviet repression, which sits in the shadow of the former KGB headquarters on Lubyanka Square in central Moscow.