WATCH: Alexei Navalny's mother, Lyudmila Navalnaya, said she was threatened by Russian officials.
Alexei Navalny's mother says she has been shown his body but Russian authorities are pressuring her to allow a “secret” burial.
In a video address, Lyudmila Navalnaya said she was brought to a morgue, where she signed a death certificate.
A medical report submitted to the former opposition leader's media secretary, Ms Navalnaya, stated that he died of natural causes.
Navalny's widow said he was killed by Russian officials.
Lyudmila Navalnaya said the law required authorities to hand over her son's body, but she was being “blackmailed” when they refused to do so. He alleged that the authorities were dictating conditions for his son's burial, including the place, time and manner of burial.
She said: “They want to take me to a new grave on the outskirts of the cemetery and say, 'Here lies your son'.”
Ms Navalnaya traveled to the northern Russian city of Salekhard following news of her son's death in a nearby penal colony six days earlier.
In a speech on Thursday, Lyudmila Navalnaya said she was being threatened by the authorities.
“They look me in the eye and tell me that if I don't agree to a secret cremation, they will do something with my son's body.”
She said investigators told her: “Time is not on your side, the body is decomposing.”
Lyudmila Navalnaya was speaking in a video posted on her late son's YouTube channel and ended the address by demanding that his body be handed over to her.
There was no immediate response from Russian officials.
Separately, US President Joe Biden met Mr Navalny's widow Yulia and his daughter Tasha Navalnaya in San Francisco on Thursday.
“The President commended Alexei Navalny for his extraordinary courage and legacy of fighting against corruption and for a free and democratic Russia,” the White House said in a statement.
Navalny died on February 16 in a prison colony. Jail officials said he fell ill following the “walk”.
The Kremlin has denied the allegations, saying Western reaction to the death was “hysterical”.
Political scientist Tatiana Stanovaya said the decision to display Ms Navalnaya's son's body was a sign the authorities wanted to force Navalny's mother to “make a deal”.
“The body was returned, but under the condition that the funeral should not become a political event,” he wrote in a post on his Telegram channel.
Airlifted to Germany, he recuperated there before returning to Russia in January 2021, where he remains in prison.
Attempts to commemorate his death have been met with a harsh response from Russian authorities, with makeshift memorials removed and hundreds arrested.
Watch: Navalny's rebuttal moments as Putin's fiercest critic
