Alexei Navalny's mother has said she has been shown his body, but added 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 on Wednesday night.
She said she had signed a death certificate. Allies of the former opposition leader said it claimed he died of natural causes.
Navalny's widow has said he was killed by Russian authorities.
Lyudmila Navalnaya said the law required authorities to hand over her son's body, but said 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, who travelled to the town of Salekhard following the news of her son's death in a nearby penal colony, also 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.
"Investigator Voropaev openly told me: 'Time is not on your side, the corpse is decomposing.'"
She 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.
Navalny died in a prison colony on 16 February.
