Russian mourners gather at the spot where opposition leader Boris Nemtsov was shot dead, near St Basil's Cathedral in Moscow, on February 28, 2015
Moscow (AFP) - Western leaders and Russia's opposition on Saturday condemned the drive-by assassination of Kremlin critic Boris Nemtsov while President Vladimir Putin suggested the crime was staged to throw suspicion on the state.
US President Barack Obama decried the "brutal" and "vicious murder" of Nemtsov, which prompted the cancellation of a major opposition rally planned for Sunday, and urged Russia to conduct an impartial probe.
French President Francois Hollande called the killing a "hateful murder" of a "defender of democracy", while British Prime Minister David Cameron said the "callous murder" must be investigated "fully, rapidly and transparently".
EU foreign policy chief Federica Mogherini, voicing "indignation" over the assassination, also called for a speedy, transparent probe.
Putin, who has taken personal charge of the investigation, was quoted by the Kremlin as saying the crime "had all the hallmarks of a contract killing and is entirely provocative in nature," suggesting it was aimed at smearing the authorities.
The last Soviet leader, Mikhail Gorbachev, also said the killing was aimed at "destabilising the situation in the country, at heightening confrontation" with the West.
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.
The brazen assassination was one of the highest-profile killings during Putin's 15 years in power and recalled the shooting of anti-Kremlin reporter Anna Politkovskaya, who was gunned down on Putin's birthday in October 2006.
Investigators said Nemtsov was shot in the back from a white car as he was walking with a woman along a bridge just metres from the Kremlin.
The Life News website identified the woman was identified as 23-year-old Ukrainian model.
One or more gunmen shot at Nemtsov at least seven or eight times, investigators said. They said one of the weapons was believed to be a Makarov pistol, used by Russian military and police.
Cartridges left at the scene came from different manufacturers, making them harder to trace, they said.
The killers evidently knew Nemtsov was planning to walk to his flat nearby, they added.
They said initial hypotheses included 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 "situation inside Ukraine".
- 'Beyond imagination' -
Speaking on radio just hours before his murder, Nemtsov sounded upbeat and urged Russians to join the 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.
The current regime has reached "a dead end in both domestic and foreign policies. They should go," Nemtsov said.
The former researcher rose to prominence as governor of the Nizhny Novgorod region in central Russia and became a vice prime minister in the late 1990s under the presidency of Boris Yeltsin.
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.
"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."
Washington led condemnation of the killing.
"We call upon the Russian government to conduct a prompt, impartial and transparent investigation into the circumstances of his murder and ensure that those responsible for this vicious killing are brought to justice," Obama said in a statement.
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."
- 'New political reality' -
A steady stream of people heaped flowers and photos of Nemtsov and set candles at the site of the murder on Saturday, with police closing off one lane of traffic to let them through.
Opposition activists have now scrapped the rally, while the authorities have permitted a march in memory of Nemtsov through the city centre.
"We are in a new political reality," said one of the organisers, Leonid Volkov.
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.