Thousands of people are expected to take to the streets of 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 will now be marching to mourn 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 are due to gather in central Moscow at 14:00 local time on Sunday, before marching to the spot on Great Moskvoretsky Bridge where Mr Nemtsov was killed.
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.
As night fell on Saturday, flowers were piled up a metre high and two metres wide on the bridge.
Placards read: "We are all Nemtsov" and "Je Suis Boris" - the latter a reference to the Je Suis Charlie messages of support following the Charlie Hebdo killings in Paris last month.
One of those attending, 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."
Opposition activist Mark Galperin said: "People are afraid to support our movement. Opposition activists receive threats every day and Boris was no exception. But they won't stop us."
Former Yabloko party leader Grigory Yavlinsky said: "The political responsibility for this murder lies with the authorities and personally President Putin."
A number of European ambassadors laid wreaths.
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.
Lawmaker Vladimir Zhirinovsky said the murder could have been connected to Nizhny Novgorod where Mr Nemtsov had been regional governor in the 1990s.
Mr Nemtsov was reportedly preparing documents on Russian military involvement in Ukraine.
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."
US President Barack Obama condemned the killing as a "brutal murder".
The Russian government must conduct a "prompt, impartial and transparent investigation", the US president urged.
He'll kill you
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.
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 PM under President Boris Yeltsin in the 1990s but fell out of favour with Mr Putin and became an outspoken opponent.
He told the weekly Sobesednik recently that his mother was worried about him.
"She is more worried about Putin than Ukraine. Every time I call her, she gives me a talking-to: 'When will you stop being rude about Putin? He'll kill you.'"
Moscow's authorities have agreed a march of up to 50,000 people can be held on Sunday.
It will begin on Kitaigorodsky Proezd at 15:00 local time and pass the site of the killing.