At least 212 people were detained at events in Russia on Friday and Saturday in memory of Alexei Navalny, Russian President Vladimir Putin's most formidable domestic opponent, who died on Friday, according to rights group OVD-Info.

It would be the largest wave of arrests at political events in Russia since Sept. 2022, when more than 1,300 were arrested at demonstrations against a "partial mobilisation" of reservists for the military campaign in Ukraine.

Navalny, a 47-year-old former lawyer, fell unconscious and died on Friday after a walk at the "Polar Wolf" Arctic penal colony where he was serving a three-decade sentence, authorities said.

OVD-Info, which reports on freedom of assembly in Russia, said at least 212 people in 21 cities across Russia had been detained at spontaneous rallies and vigils as of 1127 GMT on Saturday.

OVD-Info said that police had detained at least 109 people in St Petersburg and at least 39 in Moscow, the country's two largest cities, where Navalny's mostly educated and urban supporters had been concentrated.

The group also reported individual arrests in smaller cities across Russia, from the border city of Belgorod, where seven were killed in a Ukrainian missile strike on Thursday, to Vorkuta, an Arctic mining outpost once a centre of the Stalin-era gulag labour camps.

Footage filmed by Reuters in Moscow showed law enforcement bundling people to the ground in the snow, close to a spot where mourners had left flowers and messages in support of the dead opposition leader.

"In each police department there may be more detainees than in the published lists," OVD-Info said. "We publish only the names of those people about whom we have reliable knowledge and whose names we can publish."

Reuters could not immediately verify the count.

The hundreds of flowers and candles laid in Moscow on Friday to honour Navalny's memory were mostly taken away overnight in black bags. Russians paying their respects spoke of their despair and apathy after Navalny's death.