President Vladimir Putin has vowed to punish the killers of Kremlin critic Boris Nemtsov as Russian opposition figures denounced what they called a "political murder".
Western leaders have called for a full probe.
The 55-year-old former deputy premier, a vocal Putin critic prominent at opposition rallies, was shot in the back several times just before midnight on Friday as he walked across a bridge a stone's throw from the Kremlin walls.
The brazen assassination is the latest in a string of murders of opposition figures in Putin's 15 years in power and recalls the shooting of anti-Kremlin reporter Anna Politkovskaya, gunned down on Putin's birthday in October 2006.
Putin blamed the latest killing on foes trying to discredit the Kremlin, but vowed in a message to the victim's mother to do all he could so "the organisers and perpetrators of a vile and cynical murder get the punishment they deserve."
Allies of Nemtsov, a longtime anti-corruption crusader who served as deputy premier under Boris Yeltsin, linked his death to his opposition to the Kremlin's policies.
Hours before the killing, he had gone on a popular radio station to urge people to join an opposition rally on Sunday calling for "an immediate end to the war in Ukraine."
The rally has now been cancelled and Russia's opposition will hold a march in memory of Nemtsov through central Moscow on Sunday afternoon, crossing the bridge where he died.
"There is already a list of unsolved political murders and attacks in Russia," Amnesty International said.
"We cannot allow Boris Nemtsov to become just another name on this list."
Investigators said gunmen fired at least eight shots from a car as Nemtsov walked with a woman named by Russian media as 23-year-old Ukrainian model Anna Duritskaya, who was unhurt.
Footage aired by Russian TVC channel from a distant camera showed someone, suspected to be the killer, running along the road and jumping into a waiting car which then sped off.
The camera angle left Nemtsov and his companion hidden from view.
On Saturday, a steady stream of mourners, many in tears, filed across the bridge, heaping flowers and photos of Nemtsov at the spot where he fell.
"This was ordered by Putin, without a shadow of a doubt," said 59-year-old Alexander Badiyev.
"They have shown us what the fate will be of all those who are against them."
Opposition figures linked Nemtsov's death to his open criticisms of Putin and in particular Russia's policy on Ukraine.
They pointed the finger at a climate of hatred whipped up by the Kremlin.
Opposition leader Alexei Navalny questioned how Nemtsov could have been killed, saying he would have been under surveillance ahead of Sunday's planned rally.
US President Barack Obama condemned a "brutal" and "vicious murder" and urged a probe while British Prime Minister David Cameron said the "callous murder" must be investigated "fully, rapidly and transparently".
Putin suggested the crime was aimed at smearing the authorities.
Nemtsov feared for his life
Boris Nemtsov gave an interview this month admitting he had feared for his life over his opposition to President Vladimir Putin.
In an interview with weekly Sobesednik, Nemtsov was asked: "Have you started worrying that Putin could personally kill you in the near future or do it through middle men?"
He replied: "You know... yes. A little.
"But all the same I'm not that scared of him. If I was that afraid, I would hardly have headed an opposition party and would hardly be doing what I'm doing now," he said in the interview published in early February.
The Kremlin critic who courted public anger by opposing Russia's actions in Ukraine said he often discussed politics with his 86-year-old mother, Dina, who agreed with his views but feared he could suffer deadly consequences.
"She is categorically against what is happening in Ukraine, she thinks that it's a disaster and a complete nightmare," he said.
"But 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.' And she is being absolutely serious," Nemtsov said.
In a light-hearted exchange, the Sobesdenik journalist told Nemtsov: "I hope that common sense will prevail after all and Putin won't kill you."
"God willing. I hope so too," Nemtsov replied.
Investigators in charge of the probe into Nemtsov's killing cited threats to him over his condemnation of the murders at Charlie-Hebdo weekly as a possible motive.
Killing is a blow to democracy: Julie Bishop
Australian Foreign Minister Julie Bishop has expressed her sympathy for the family of Russian opposition leader
Boris Nemtsov, saying his death has all the hallmarks of a political assassination.
"This is a tragic, senseless killing. There are many who say it has all the hallmarks of a political assassination," Ms Bishop told Network Ten on Sunday.
"It's a tragedy for Russia. It's a tragedy for democracy."
She said there should be a detailed investigation into his death.
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