Alexei Navalny's mother said she has been shown her son’s body and a death certificate which says he died from natural causes.
Last week Russian authorities said the opposition leader died suddenly in an Arctic penal colony but his team and family allege that the Kremlin murdered him - an allegation the Kremlin has rejected.
In a video address from Russia on Thursday afternoon, Lyudmila Navalnaya said Russian investigators had been "blackmailing" her over the funeral of Mr Navalny.
In a YouTube video, she accused the Kremlin of planning a "secret" funeral for her son 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.
The mother had travelled to the prison where her son was being held and issued a video appeal to Putin to hand over her son.
She had also filed a lawsuit seeking the release of the body, but Russian state media have said her complaint will not be heard by a court until March 4.
Rights organisation OVD-Info said more than 83,000 people had signed a petition drafted to Russia's Investigative Committee, urging it to inform Mr Navalny's relatives where his body is and to hand it over to them.
At least 800 people have signed a separate petition initiated by a group of Russian Orthodox priests, saying his family has the right to say farewell to Mr Navalny and give him a Christian burial.
The petition urged the authorities to respect Mr Navalny's memory, not only as an opposition politician but as a believer.
"Do not overshadow the tragedy of his death by refusing such a simple and humane request. Remember that everyone is equal before God," it said.
Also on Thursday, Mr Navalny's wife Yulia said that she and her daughter Dasha were together and comforting each other.
Yulia Navalnaya posted a photograph on X showing her and Dasha huddled together in mutual support.
Милая моя девочка. Я же прилетела тебя обнять и поддержать, а ты сидишь и поддерживаешь меня.
— Yulia Navalnaya (@yulia_navalnaya) February 22, 2024
Такая сильная, смелая и стойкая. Мы обязательно со всем справимся, родная. Как хорошо, что ты рядом. Люблю. pic.twitter.com/FzDbrykUjH
His wife was shown looking into the camera with her hand over her mouth, and Dasha wrapped in a black coat and gazing sadly into space.
"My dear girl. I flew here to hug you and support you, and you are sitting and supporting me. So strong, brave and resilient. We will definitely cope with everything, my dear one. It's so good that you're at my side. I love you," she wrote.
It was not clear where and when the picture was taken. Ms Navalnaya, 47, was in Munich last week when she learned of the death of her husband, Russia's best known opposition leader in the West.
Dasha, their older child, is a student at Stanford University in the United States. They also have a son, Zakhar.
Ms Navalnaya has pledged to continue her husband's work and blamed President Vladimir Putin for his death.
Putin has not commented publicly on the death of Mr Navalny, who was serving sentences totaling more than 30 years on a series of charges including fraud and extremism, which he said were trumped up to silence him.
The Kremlin has said it was not involved in his death, the circumstances of which it says remain under investigation.
