Alexei Navalny's mother Lyudmila has accused Russian investigators of planning to bury her son in secret without a funeral, and said she would not agree to it.
"They want this to be done secretly, with no farewell. They want to bring me to the edge of a cemetery, to a fresh grave and say: here lies your son. I don't agree to this," she said in a YouTube video.
There was no immediate response from Russian investigators.
Mr Navalny, 47, Russia's best-known opposition politician, died suddenly in an Arctic penal colony last week. His aides and family have alleged that the Kremlin murdered him, an allegation the Kremlin has rejected.
His mother said she had been shown his body and death certificate. Mr Navalny's aides said the death certificate stated that the opposition politician had died of natural causes.
Mr Navalny's mother said she had been taken to a morgue yesterday evening to see his body.
"The investigators claim that they know the cause of death, they have all the medical and legal documents ready, which I saw, and I signed the medical death certificate," she said, dressed in black and speaking in a calm voice.
"According to the law, they should have given me Alexei's body right away, but they have not done so until now. Instead, they are blackmailing me, setting me conditions on where, when and how Alexei should be buried. This is illegal."
Mr Navalny's mother said: "I'm recording this video because they started threatening me. Looking into my eyes, they say that if I don’t agree to a secret funeral, they will do something with my son’s body."
She quoted one of the investigators as saying: "Time is not on your side, corpses decompose."
"I don't want special conditions," she said. "I just want everything to be done according to the law. I demand that my son's body be returned to me immediately."
The Kremlin has said it had nothing to do with Mr Navalny's death, and that the circumstances are being investigated. President Vladimir Putin has yet to comment on it.
Biden met with Navalny's widow, daughter
US President Joe Biden has said he met privately in California with the widow and daughter of Mr Navalny.
After meeting with Yulia and Dasha Navalnaya in San Francisco, Mr Biden told reporters that the late opponent to President Vladimir Putin was "a man of incredible courage".
He said that Yulia Navalnaya and her daughter, who studies at Stamford University in California, "are emulating that".
The widow is "going to continue the fight," he said. "She's not giving up."
Mr Biden also reaffirmed a plan to unveil sanctions tomorrow, saying they would be "against Putin, who is responsible for his death".
A White House statement earlier said that Mr Biden had used his meeting behind closed doors to express "admiration for Alexei Navalny's extraordinary courage and his legacy of fighting against corruption and for a free and democratic Russia".
Mr Biden "emphasised that Alexei's legacy will carry on through people across Russia and around the world mourning his loss and fighting for freedom, democracy and human rights".
US National Security Council spokesman John Kirby told journalists that Russian authorities should return the campaigner's body to his mother so that she can "properly memorialise... her son's bravery and courage and service".
Earlier, the US government marked the upcoming two-year anniversary of Russia's invasion of pro-Western Ukraine by unsealing charges against a series of wealthy Russians to help cut the "flow of illegal funds that are fueling" Moscow's war.
