Aleksei A. Navalny’s political allies on Saturday confirmed his death, saying that his mother, Lyudmila Navalnaya, had received an official notification that her son died on Friday afternoon. The confirmation came as Russian officials detained hundreds of people mourning his death.

Mr. Navalny’s family was still waiting for officials to release the body. Kira Yarmysh, Mr. Navalny’s spokeswoman, said in a statement on X that Russian investigators had transferred Mr. Navalny’s body from a penal colony in the Arctic to the town of Salekhard, where it was being examined.

“We demand for Aleksei Navalny’s body to be released to his family immediately,” Ms. Yarmysh said in her statement.

The detentions of Mr. Navalny’s mourners signaled that the Kremlin wanted to prevent his death from turning into a political event. At least 340 people in 30 cities had been detained since the death was announced on Friday, according to OVD Info, a rights group that tracks such arrests.

In Moscow, hundreds of Russians came to the Wall of Grief monument honoring victims of political repression in Russia. They were met with a heavy police presence. Overnight, Moscow city authorities removed piles of flowers that people had left to commemorate Mr. Navalny.

Mr. Navalny’s mother and one of his lawyers arrived on Saturday morning at the penal colony in the Kharp settlement, Ms. Yarmysh said in a video statement. She and others had to wait for two hours before a prison official came out to say that Mr. Navalny’s body had been transferred to Salekhard.

Ivan Zhdanov, the head of Mr. Navalny’s Anti-Corruption Foundation, said in a post on X that they were told in the penal colony that he died because of the “sudden death syndrome.”

In Salekhard, a regional center close to the penal colony, Mr. Navalny’s mother and his lawyer found the local morgue closed. When they called, the officials said that Mr. Navalny’s body was not there, Ms. Yarmysh said. Another lawyer was told, she said, that an “additional histology” had been performed to determine the cause of Mr. Navalny’s death, and that its results should be ready next week.

“They lie and they do everything not to give out the body,” Ms. Yarmysh said.

In a separate video address, Ms. Yarmysh accused Russia’s president, Vladimir V. Putin, of killing Mr. Navalny.

“Three and a half years ago Putin tried to kill Aleksei,” Ms. Yarmysh said, referring to Mr. Navalny’s poisoning with a nerve agent in 2020. “Yesterday, he killed him.”

Ms. Yarmysh is a member of a team of Mr. Navalny’s allies. Working from outside Russia, they have continued to carry out his work after his poisoning and his subsequent imprisonment, publishing his statements and organizing political events.

Prison authorities said Mr. Navalny fell unconscious and died after a walk at the penal colony, where he had been transferred at the end of December.

In their statement about his death, the prison authorities said that its causes were “being determined.” Local investigators said that they had begun a “procedural check” into Mr. Navalny’s death. They said “a set of investigative and operative measures is being carried out aimed at establishing all circumstances of the incident.”

Yevgeny Smirnov, a Russian lawyer, said that means a forensic medical examination is probably being conducted on Mr. Navalny’s body. Relatives can retrieve the body for burial only after the cause of death is officially determined, Mr. Smirnov said.

According to Public Verdict, a Russian rights group, such medical examinations usually give only very general causes of death.

“Such investigations absolutely cannot be trusted,” the group said in a statement.

Upon retrieving the body, relatives can bury it at a cemetery, the group said. Mr. Navalny’s family members have not commented on the possible burial arrangements.