The hundreds of flowers and candles laid in Moscow on Friday to honour the memory of Alexei Navalny, Russia's most prominent opposition leader, were taken away overnight in black bags.

Russia's prison service said that Navalny, 47, fell unconscious and died on Friday after a walk at the "Polar Wolf" Arctic penal colony. Navalny's spokeswoman Kira Yarmysh on Saturday confirmed his death, citing an official notice given to his mother, Lydumila.

In central Moscow, several dozen roses and carnations remained in the softening snow on Saturday at the monument to the victims of Soviet repression, which sits in the shadow of the former KGB headquarters on Lubyanka Square.

Vladimir Nikitin, 36, was alone laying a carnation at the Solovetsky Stone, which hails from the islands with the same name in the White Sea where one of the first "Gulag" forced labour camps was founded in 1923 by the Bolsheviks.

Policemen looked on.

When asked for an interview by Reuters, Nikitin asked to speak in the underpass which threads beneath Lubyanka Square, citing the fear of detention.

"Navalny's death is terrible: hopes have been smashed," Nikitin said.

"Navalny was a very serious man, a brave man and now he is no longer with us. He spoke the truth - and that was very dangerous because some people didn't like the truth."

At the "Wall of Sorrow" memorial on the avenue named after Soviet physicist and dissent Andrei Sakharov, some Russians laid flowers beside pictures of Navalny. One message read: "We will not forget, nor shall we forgive."

"I came because I have grief," said Arkady, who declined to give his second name. "He was a man who I respected. I had hopes that he was someone who could do something in the future."

The West, including US President Joe Biden, blamed President Vladimir Putin for the death. Western leaders did not cite evidence.

Kremlin spokesman Dmitry Peskov said the reaction of Western leaders to the death was unacceptable and "absolutely rabid".

Russian authorities viewed Navalny and his supporters as extremists with links to the CIA intelligence agency who are seeking to destabilise Russia. They have outlawed his movement, forcing many of his followers to flee abroad.

The death of Navalny, a former lawyer, robs the disparate Russian opposition of its most charismatic and courageous leader as Putin prepares for an election that will keep the former KGB spy in power until at least 2030.