The mother of Alexei Navalny, Russian President Vladimir Putin's main opponent who died in an Arctic prison last week, said Thursday officials are pressuring her into a "secret" burial for her son.

Lyudmila Navalnaya said she had been able to see the body in the morgue in Salekhard, the nearest town to the remote prison, after several days of being barred from the morgue.

"Yesterday evening they secretly took me to the morgue where they showed me Alexei," she said in a video released on social media by Navalny's team.

But she said of the burial: "They want it to be done secretly without a chance to say goodbye."

"They are blackmailing me, they put conditions for where, when and how Alexei should be buried. This is illegal," she said.

"They want to take me to the edge of a cemetery to a fresh grave and say: Here is where your son lies. I am against that.

"I want that for those of you for whom Alexei is dear, for everyone for whom his death became a personal tragedy, to have the possibility to say goodbye to him."

She said she recorded the video because investigators were "threatening" her.

"Looking me in the eye, they said that if I do not agree to a secret funeral they'll do something with my son's body... I ask for my son's body to be given to me immediately," she said.

Navalny, an anti-corruption crusader and leading domestic foe of Russian President Vladimir Putin, died last Friday aged 47 in an Arctic prison.

His wife Yulia Navalnaya has accused Putin of having murdered her husband.