The death of jailed Russian opposition leader Alexei Navalny is sending ripples across Russian society as his mother is still unable to see his remains.

Navalny was a leading critic of President Vladimir Putin.

Authorities announced on Friday that he died in the prison where he was serving a jail sentence, but they have yet to disclose the details about the cause of his death.

A senior member of Navalny's support group says his mother and lawyer were told at the prison that he died of "sudden death syndrome."

But they have not been able to see Navalny's remains to find out for themselves the cause of his death.

Independent Russian newspaper Novaya Gazeta Europe quoted an inmate at the prison as saying that a "mysterious commotion" broke out there on Thursday evening, before the Friday's announcement of his death.

The inmate reportedly said security was suddenly tightened, and that inmates heard vehicles drive onto the prison grounds.

The newspaper reported that Navalny's body was transferred to the regional capital of Salekhard of Russia's far northern Yamalo-Nenets autonomous district.

The paper also said his body reportedly had bruises. A paramedic suggested one on his chest may have come from a heart massage, but it is not clear why he had a cardiac arrest.

Meanwhile, as of noon Monday, over 50,000 appeals had been sent to a Russian human rights group, demanding that the country's Investigative Committee hand over Navalny's body to his family.