Alexei Navalny’s mother has said she has been shown his body, but that the Russian authorities were pressuring her to allow a “secret” burial.
In a video address, Lyudmila Navalnaya said she had been brought to a morgue where she signed a death certificate.
The former opposition leader’s press secretary said a medical report presented to Ms Navalnaya stated he died of natural causes.
Navalny’s widow has said he was killed by Russian authorities.
Lyudmila Navalnaya said the law required officials to hand over her son’s body, but that she was being “blackmailed” as they refused to do so. She alleged authorities were setting conditions for the burial of her son, including the place, time and manner of his burial.
She said: “They want to take me to the outskirts of the cemetery to a fresh grave and say: ‘Here lies your son.'”
Ms Navalnaya travelled to the northern Russian town of Salekhard following the news of her son’s death in a nearby penal colony six days ago.
She has previously been denied access to her son’s body, on Tuesday appealing personally to Russian President Vladimir Putin to allow her to bury him.
In Thursday’s address, she said she was being threatened by the authorities.
“Looking into my eyes, they say that if I do not agree to a secret funeral, they will do something with my son’s body.”
She said she was told by investigators: “Time is not on your side, the corpse is decomposing.”
Lyudmila Navalnaya was speaking in a video posted to her late son’s YouTube channel, and finished the address by demanding his body be returned to her.
There was no immediate response from Russian authorities.
Political scientist Tatiana Stanovaya said that the decision to show Mrs Navalnaya her son’s body showed authorities wanted to persuade Navalny’s mother to “make a deal.”
“The body is given back, but under the condition that the funeral does not become a political event,” she wrote in a post on her Telegram channel.
Navalny died in a prison colony on 16 February. Prison officials said he had fallen ill following a “walk”.
But his widow Yulia Navalnaya has claimed that he was killed on the orders of Mr Putin and vowed to continue his struggle.
The Kremlin has denied the allegations, calling Western reaction to the death “hysterical”.
In August 2020, the former opposition leader was poisoned using the Novichok nerve agent by a team of would-be assassins from the Russian secret services.
Airlifted to Germany, he recovered there before returning to Russia in January 2021, where he was imprisoned.
Attempts at commemorating his death have been met by a heavy-handed response from Russian authorities, with makeshift monuments cleared and hundreds arrested.
