Boris Nemtsov, a charismatic Russian opposition leader and sharp critic of President Vladimir Putin, was gunned down near the Kremlin, just a day before a planned protest against the government.

Nemtsov's death late on Friday ignited a fury among opposition figures who assailed the Kremlin for creating an atmosphere of intolerance of any dissent and called the killing an assassination.

Putin quickly offered his condolences and called the murder a provocation. He also vowed to do everything possible to bring to justice those responsible for the vile murder of Nemtsov.

"Everything will be done so that the organisers and perpetrators of a vile and cynical murder get the punishment they deserve," Putin said in a telegram to Nemtsov's mother published on the Kremlin's website.

Putin told Nemtsov's 86-year-old mother, Dina Eidman, that his death was an irreparable loss and that he had "left his trace in Russia's history, in politics and public life."

Nemtsov, a 55-year-old former deputy prime minister, was working on a report presenting evidence that he believed proved Russia's direct involvement in the separatist rebellion that has raged in eastern Ukraine since last April.

Ukraine and the West accuse Russia of backing the rebels with troops and sophisticated weapons. Moscow denies the accusations.

'Contract killing'

Hundreds of mourners gathered at a makeshift memorial at the site of the killing on Saturday in tribute to the slain opposition politician.

Putin ordered Russia's top law enforcement chiefs to personally oversee the probe of Nemtsov's killing.

"Putin noted that this cruel murder has all the makings of a contract hit and is extremely provocative," presidential spokesman Dmitry Peskov said in remarks carried by Russian news agencies.

President Barack Obama called on Russia's government to perform a "prompt, impartial and transparent" investigation to bring the perpetrators to justice. Obama called Nemtsov a "tireless advocate" for the rights of Russian citizens.

Nemtsov assailed the government's inefficiency, rampant corruption and the Kremlin's Ukraine policy, which has strained relations between Russia and the West to a degree unseen since Cold War times.

In an interview with the Sobesednik newspaper, Nemtsov said earlier this month that his 86-year-old mother was afraid that Putin could have him killed for his opposition activities.

Asked if he had such fears himself, he responded by saying: "If I were afraid I wouldn't have led an opposition party."

Speaking on radio just a few hours before his death, he harshly criticised Putin for plunging Russia into the crisis by his "mad, aggressive and deadly policy of war against Ukraine".

"The country needs a political reform," Nemtsov said on Ekho Moskvy radio. "When power is concentrated in the hands of one person and this person rules for ever, this will lead to an absolute catastrophe, absolute."

Ukrainian President Petro Poroshenko called Nemtsov a personal friend and a "bridge" between the two countries. He said on his Facebook page that he hopes the killers will be punished.

Nemtsov's lawyer Vadim Prokhorov said the politician had received threats on social networks and told police about them, but authorities didn't take any steps to protect him.

The Russian interior ministry, which oversees Russia's police force, said that Nemtsov was killed by four shots in the back from a passing car as he was walking over a bridge just outside the Kremlin shortly after midnight.

Drive-by shooting

Interior ministry spokeswoman Yelena Alexeyeva told reporters that Nemtsov was walking with a female acquaintance, a Ukrainian citizen, when a vehicle drove up and unidentified assailants shot him dead.

The woman wasn't hurt and was being questioned by police.

Mikhail Kasyanov, a former Russian prime minister now also in opposition, said he was shocked.

"In the 21st century, a leader of the opposition is being demonstratively shot just outside the walls of the Kremlin!" Kasyanov told reporters as Nemtsov's body, placed in a plastic bag, was removed on a rainy and cold night, as the Kremlin bells chimed nearby.

"The country is rolling into the abyss."

Sunday's rally was pushed to the city's outskirts by the authorities, but Kasyanov said the rally organisers decided that they will stage a demonstration in the centre of the capital to commemorate Nemtsov.

Officials' failure to authorise it would be certain to cause anger and could lead to unrest.

Opposition activist Ilya Yashin, who last spoke to Nemtsov two days before the killing, said he had no doubt that Nemtsov's murder was politically motivated.

"Boris Nemtsov was a stark opposition leader who criticised the most important state officials in our country, including President Vladimir Putin. As we have seen, such criticism in Russia is dangerous for one's life," Yashin said.