According to BBC, thousands of people are marching in central Moscow to honour opposition politician Boris Nemtsov, who was shot dead on Friday.
He was due to lead an opposition march on Sunday but his supporters are instead mourning his death.
President Vladimir Putin condemned Mr Nemtsov’s murder as “vile and cynical” and vowed to find the killers.
Mr Nemtsov’s allies call it a political killing linked to his opposition to Mr Putin and the Ukraine conflict.
Opposition supporters have been gathering at a point not far from the Kremlin. The first marchers are on their way to the spot on Great Moskvoretsky Bridge where Mr Nemtsov was killed.
Several thousand people are also marching in St Petersburg.
Moscow city authorities had previously approved a march for up to 50,000 people but organisers said more people might now attend following the murder.
Many people are carrying the national flag and flowers to lay at the scene of the killing, which is always piled high with tributes. A few were holding Ukrainian flags.
Others are holding placards with slogans such as “He died for the future of Russia” and “They were afraid of you, Boris”.
People have been paying their respects at the site since early on Saturday.
One of those attending on Saturday, Alexander Badiyev, said: “This was ordered by Putin, without a shadow of a doubt. They have shown us what the fate will be of all those who are against them.”
Former Yabloko party leader Grigory Yavlinsky said: “The political responsibility for this murder lies with the authorities and personally President Putin.”
Former UK ambassador to Russia, Sir Tony Brenton, told the BBC there was now “an atmosphere of political hysteria in Russia”.
“And that is an atmosphere which has generated a lot of really rather nasty right-wing ex-soldiers’ groups to come to the surface. And it is quite possible that it is a group like that that was responsible for Boris’s death.”
Russia’s Investigative Committee said it was looking into a number of possible motives, including Mr Nemtsov’s opposition to the Ukraine war, his political and personal life, Islamic extremism or an attempt to destabilise the state.
A number of pro-government figures suggested Mr Nemtsov had been made a sacrificial victim to show the state in a bad light.
Ramzan Kadyrov, the Putin-backed leader of Chechnya, blamed “Western special services, trying by any means to create internal conflict in Russia”.
Presidential spokesman Dmitry Peskov said Mr Putin had noted “that this cruel murder has all the makings of a contract hit and is extremely provocative”.
Others suggested there could have been personal enmity over his private or business life.
Mr Nemtsov was reportedly preparing documents on Russian military involvement in Ukraine in the weeks before his death.
Ukrainian President Petro Poroshenko said: “Boris had declared he would provide clear evidence of Russian armed forces’ participation in [the war] in Ukraine. Somebody was afraid of this… They killed him.”
Mr Nemtsov, 55, had been dining at a restaurant with his girlfriend Anna Duritskaya on Friday night.
They left together to walk to his flat, crossing the bridge, where a white car drew up and Mr Nemtsov was shot four times with a pistol at around 23:40 (20:40 GMT).
Footage on Russian TV showed a white Lada Priora car in the area but there was no confirmation it was the one involved. One shot showed someone running along the road and jumping into the waiting car, which sped off.
In a telegram to Mr Nemtsov’s mother, published on the Kremlin website, Mr Putin vowed to bring the killers to justice. He praised Mr Nemtsov’s openness and honesty.
Mr Nemtsov served as first deputy prime minister under President Boris Yeltsin in the 1990s but fell out of favour with Mr Putin and became an outspoken opponent.