Alexei Navalny's mother, who was shown the body, said Russian authorities were pressuring her to allow a “secret” burial.
Lyudmila Navalnaya said in a video address that her body was taken to the morgue, where she signed the death certificate.
A spokesman for the former opposition leader said the medical report submitted to Navalnaya said he died of natural causes.
Navalny's widow said he was killed by Russian authorities.
Lyudmila Navalnaya said she was “blackmailed” because authorities refused to hand over her son's body to the authorities, which is required by law. She claimed that authorities were setting conditions such as the place, time and manner of her son's burial.
“They want to take me to a new grave on the outskirts of the cemetery and say, 'Here lies your son,'” she said.
Navalnaya traveled to the northern Russian town of Salekhard six days ago after receiving word that her son had died in a nearby penal colony.
In a speech Thursday, she said she had received threats from authorities.
“They look me in the eye and say that if I don't agree to a secret funeral, they will do something to my son's body.”
She said investigators told her, “Time is not on your side. The body is decomposing.”
Lyudmila Navalnaya gave a speech in a video posted on her late son's YouTube channel, ending her speech by demanding the return of the body.
There was no immediate reaction from Russian authorities.
Political scientist Tatyana Stanovaya said the decision to show Navalny her son's body showed authorities wanted to persuade his mother to make a “deal.”
“The body will be returned, but on the condition that the funeral does not become a political event,” she said in a post on her Telegram channel.
Navalny died in prison on February 16. According to prison officials, the man became ill after taking a “walk.”
The Kremlin denied the allegations and called the Western reaction to the death “hysterical.”
He was airlifted to Germany, where he recovered, before returning to Russia in January 2021, where he was imprisoned.
Efforts to commemorate his death were met with a heavy-handed response from Russian authorities, with makeshift memorials removed and hundreds arrested.