MOSCOW - President Vladimir Putin vowed to punish the killers of Kremlin critic Boris Nemtsov as Russian opposition figures denounced what they called a "political murder" and Western leaders 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 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 Saturday 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", adding that Putin should resign.
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.
A steady stream of mourners, many in tears, filed across the bridge Saturday, heaping flowers, candles 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." 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 the suspected 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.
"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."
'Tragedy for democracy'
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.
"The political responsibility for this murder lies with the authorities and personally President Putin - those who started and are fighting a war and are running a propaganda campaign of hatred in its support," former Yabloko liberal party leader Grigory Yavlinsky wrote on Facebook.
Opposition leader Alexei Navalny questioned how Nemtsov could have been killed, saying he would have been under surveillance ahead of Sunday's planned rally.
"I can't believe that yesterday night he could have strolled towards the Kremlin without being watched," he wrote on his blog.