Moscow: President Vladimir Putin on Saturday vowed to bring the "vile" killers of Kremlin critic Boris Nemtsov to justice as Western leaders and Russia`s opposition roundly condemned the drive-by shooting near the Kremlin walls.
Putin, who earlier blamed the assassination on foes trying to discredit the Kremlin, said in a message to Nemtsov`s mother: "Everything will be done so that the organisers and perpetrators of a vile and cynical murder get the punishment they deserve,"
The 55-year-old former deputy premier who was a vocal critic of Putin, was shot in the back several times shortly before midnight Friday as he walked across a bridge a stone`s throw from the Kremlin.
Hours later on Saturday a steady stream of mourners, many in tears, filed by on the bridge, heaping flowers and photos of Nemtsov at the spot where he fell.
Police closed off a lane of traffic to let them through.
Investigators said Nemtsov was shot in the back while walking with a woman, who was not injured. She was later identified as 23-year-old Ukrainian model, Anna Duritskaya, by Russian state television.
The murder, investigators added, was the work of one or more gunmen who shot seven or eight times at the opposition figure who in the 1990s served as deputy premier under then president Boris Yeltsin.
He "left his trace in Russia`s history, in politics and public life," Putin said in the message to his 86-year-old mother, Dina Eidman.
"He always directly and honestly announced his position, stood up for his point of view," Putin added.
Prime Minister Dmitry Medvedev praised Nemtsov as a "principled person" who "acted openly, consistently and never betrayed his views."
Hours earlier the Putin critic and anti-corruption crusader had called on Russians to join an opposition rally on Sunday against the Kremlin stance in Ukraine.
US President Barack Obama condemned a "brutal" and "vicious murder" and urged a probe and British Prime Minister David Cameron said the "callous murder" must be investigated "fully, rapidly and transparently".
The brazen assassination is one of the highest-profile killings in Putin`s 15 years in power and recalls the shooting of anti-Kremlin reporter Anna Politkovskaya, who was gunned down on Putin`s birthday in October 2006.Earlier, Putin and other officials suggested the crime was aimed at smearing the authorities. It "had all the hallmarks of a contract killing and is entirely provocative in nature," he was quoted as saying by the Kremlin.
Putin`s spokesman Dmitry Peskov told Kommersant FM radio station that the crime "can look very much like a provocation" since "Boris Nemtsov was known as being in opposition to the Russian leadership."
Even the last Soviet leader Mikhail Gorbachev believed the killing was aimed at "destabilising the situation in the country, at heightening confrontation" with the West.
And the powerful Investigative Committee leading the probe said it was looking into a possible "provocation to destabilise the political situation in the country."
Nemtsov could have been "offered as a sacrifice" by those who are "not averse to using such a method to reach their political aims," it suggested.
Investigators said the murder weapon was believed to be a Makarov pistol, used by Russian military and police.
The killers evidently knew Nemtsov was walking to his flat nearby, they added.
They listed as initial hypotheses a link to "Islamist extremism" and the Charlie Hebdo massacre, noting that Nemtsov had received threats after he condemned the killings in Paris as well as the "situation inside Ukraine".Hours before his murder, Nemtsov had urged Russians to join a planned opposition rally on Sunday.
"The key political demand is an immediate end to the Ukraine war," he said on popular Echo of Moscow radio, adding that Putin should quit.
Opposition activists scrapped the rally after news of his death while the authorities gave permission for a march in memory of Nemtsov through the city centre on Sunday afternoon.
Ukrainian President Petro Poroshenko called Nemtsov a "bridge between Ukraine and Russia".
"The murderers` shot has destroyed it. I think it is not by accident."
"This is payback for the fact that Boris consistently, for many, many years fought for Russia to be a free democratic country," opposition leader Mikhail Kasyanov, who served as prime minister under Putin, told reporters after visiting the murder scene.
"In the 21st century, in 2015, a leader of the opposition is shot dead by the Kremlin walls. It is beyond imagination."
The former researcher rose to prominence as governor of the Nizhny Novgorod region in central Russia before serving in Yeltsin`s team.
After leaving parliament in 2003, he led several opposition parties and groups.
A passionate orator with a rock star image and popular with women, Nemtsov was a key speaker at mass opposition rallies against Putin`s return to the Kremlin in 2012.
He wrote a series of reports critical of corruption and misspending under Putin.
In 2013, he said up to $30 billion of the estimated $50 billion assigned to the Olympic Games that Russia was to host in Sochi had gone missing.
The Kremlin has denied the claims.
Alexei Venediktov, editor-in-chief of Echo of Moscow radio, wrote that Nemtsov, who leaves behind four children and an elderly mother, knew he was taking risks by openly criticising Putin.
"But I will not leave Russia, who would fight then?" he quoted the veteran politician as saying.