Alexei Navalny’s mother has been unable to recover his body after his death in an Arctic jail, a close aide to the dead Russian opposition leader says.
Kira Yarmysh said his mother, Lyudmila, was told his body would only be handed over once a post-mortem examination had been completed.
Navalny’s team believes the anti-corruption campaigner was murdered on the orders of President Vladimir Putin.
A rights group said 400 Russians had been arrested for laying tributes.
Western governments say the blame lies with Russian authorities for the 47-year-old’s sudden death, while foreign ministers from the G7 group of rich countries called on Russia to “urgently clarify” the circumstances surrounding it.
Mr Putin has not publicly commented since the Russian prison service announced on Friday that Navalny had been taken ill and died at the remote IK-3 prison in the Arctic Circle.
In the immediate aftermath, the Kremlin said it was aware and the president had been informed.
Russia’s Foreign Ministry said it rejected “biased and unrealistic” assessments over his cause of death made during a meeting with British officials on Saturday.
Navalny was one of the most prominent faces of Russian opposition to Mr Putin’s regime and was serving a three-decade sentence for politically motivated charges at the “Polar Wolf” penal colony in Kharp, about 1,200 miles (1,900 km) north of Moscow.
His mother, Lyudmila Navalnaya, was reportedly told by the prison service he died on Friday after collapsing and falling unconscious during a walk, his team said.
She visited the colony on Saturday and was given an official notice stating the time of death as 14:17 local time (09:17 GMT), Ms Yarmysh said.
Another Navalny ally, Ivan Zhdanov, said the activist’s mother was told the cause of death was said to be “sudden death syndrome” – a generic, vague term for a condition which could cover a cardiac arrest with no apparent cause.
His team said that Ms Navalnaya was told his body had been taken to the town of Salekhard, near the prison complex, but when she arrived the morgue was closed.
Prison officials reportedly told her an initial post-mortem examination was inconclusive and a second would have to be carried out.
Navalny’s allies claim his body is purposely being withheld by the Russian authorities so they can “cover traces”, and call for the body to be returned to his family “immediately”.
Meanwhile, more than 400 people have been arrested following vigils and gatherings across Russia, according to independent Russian human rights monitoring group OVD-Info.
OVD-Info, which reports on freedom of assembly in Russia, said arrests had taken place in 36 cities, with the largest numbers in the capital Moscow and St Petersburg.
On Saturday, police in Moscow detained about 15 people who had laid flowers and lit candles at the foot of the “Wall of Grief” monument to the victims of repression during the Soviet-era.
Protests are also being held near Russian embassies in many countries.
G7 foreign ministers meeting at the Munich Security Conference on Saturday observed a minute’s silence to pay tribute to the Russian activist.
British Foreign Secretary Lord Cameron said the UK would be “taking action”.
Also in Munich was Ukrainian President Volodymyr Zelensky, who described Mr Putin as a “thug” and said it was “absurd” to perceive him as the “legitimate head of a Russian state”.
Navalny had been an outspoken critic of Russia’s full-scale invasion of Ukraine, which began two years ago next week.
