MOSCOW - Boris Nemtsov, a Russian opposition politician and former deputy prime minister, has been shot dead in central Moscow, the Interior Ministry said early on Saturday. Nemtsov, 55, an outspoken critic of President Vladimir Putin, had been due to take part on Sunday in the first big opposition protest in months in the Russian capital.

“Nemtsov BE died at 2340 hours as a result of four shots in the back,” an Interior Ministry spokeswoman said by telephone. President Vladimir Putin vowed to bring the “vile” killers of Nemtsov to justice.

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,”

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.” Earlier, Putin and other officials suggested the crime was aimed at smearing the authorities.

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.” The Kremlin said Putin had taken personal control of the investigation.

Western leaders condemned the drive-by shooting of Russian opposition leader Boris Nemtsov.

US President Barack Obama decried the “brutal” and “vicious murder” of Nemtsov, which came ahead of a major opposition rally planned for Sunday, and called on Russia to conduct an impartial probe.

French President Francois Hollande called the killing a “hateful murder” of a “defender of democracy”.

The last Soviet leader Mikhail Gorbachev, quoted by Interfax, said the killing was aimed at “destabilising the situation in the country, at heightening confrontation” with the West.

A constant stream of people laid flowers and set candles at the site of the murder on Saturday morning, with police closing off one lane of traffic to let them through, an AFP reporter saw.

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 2006.

Investigators said Nemtsov was shot by unidentified assailants from a white car as he was walking with a woman along a bridge just metres from the Kremlin. The woman was identified as Ukrainian model Anna Duritskaya by Life News website.

“According to preliminary information, an unidentified person shot at Boris Nemtsov no fewer than seven or eight times from a car as he was walking along the Bolshoi Moskvoretsky bridge,” said investigators.

An AFP reporter saw blood on the rain-soaked pavement on the side of the bridge near the red walls of the Kremlin, and roses lying beside a police barrier at the scene.

Speaking on radio just hours before his murder, Nemtsov sounded upbeat and urged Russians to join a major opposition rally planned for 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,” said Nemtsov, who reportedly was compiling a report on Russia’s involvement in the Ukraine conflict.

After working as a research scientist in the late Soviet era, Nemtsov 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.”

Ukrainian President Petro Poroshenko, writing on Facebook, called Nemtsov a “bridge between Ukraine and Russia”. “The murderers’ shot has destroyed it. I think it is not by accident.” Thorbjorn Jagland, secretary general of the Council of Europe, added that he was ‘shocked and appalled’. “Killers must be brought to justice,” he said on Twitter. Exiled opposition leader Mikhail Khodorkovsky said his family was grieving.

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.

Former world chess champion Garry Kasparov, a highly vocal critic of Putin, wrote on Twitter: “(Whether) Putin gave order to murder Boris Nemtsov is not the point. It is Putin’s dictatorship. His 24/7 propaganda about enemies of the state.”