Russian opposition leader Boris Nemtsov, a fierce critic of President Vladimir Putin, has been gunned down while walking in sight of the Kremlin, prompting an international chorus of condemnation.
The murder of the 55-year-old politician, an outspoken critic of Russia's involvement in Ukraine, came before a major opposition rally he was set to lead this weekend and sent shock waves across the country.
US President Barack Obama led condemnation of the "brutal" and "vicious murder" of Nemtsov, whom he had met on a visit to Moscow, and called on Russia to conduct an impartial investigation of the killing.
Putin's spokesman Dmitry Peskov said the Russian leader had taken the investigation "under personal control".
The murder "bears the hallmarks of a contract killing", he said on radio, describing it as a provocation.
Police and investigators said Nemtsov was shot by unidentified assailants as he was walking with a woman along a bridge just metres from the Kremlin.
"According to preliminary information, an unidentified person shot at Boris Nemtsov no fewer than 7-8 times from a car as he was walking along the Bolshoi Moskvoretsky bridge," investigators said in a statement.
Interior ministry spokeswoman Yelena Alekseyeva said Nemtsov was hit by four bullets.
An Agence France-Presse reporter saw a blood stain on the pavement on the side of the bridge near the red walls of the Kremlin, with roses laid by a police barrier at the scene.
In one of his last tweets, Nemtsov urged Russians to take to take to the streets and join a major opposition rally planned for Sunday.
"Are you tired of lies, propaganda, war and price hikes?" he wrote on Twitter. "Then come to the 'Spring March'."
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 deputy prime minister in the late 1990s under the presidency of Boris Yeltsin.
After leaving parliament in 2003, he led several opposition parties and groups.
An accomplished orator with a rock-star image and popular with women, Nemtsov was one of the key speakers at mass opposition rallies against Putin's return to the Kremlin in 2012.
He developed a reputation for writing critical reports about corruption and other misspending under Putin.
In 2013, he said up to $US30 billion ($A38.50 billion) of the estimated $US50 billion assigned to the Olympic Games Russia was to host in Sochi had gone missing.
The brazen killing in central Moscow recalled the high-profile shooting of anti-Kremlin reporter Anna Politkovskaya, who was gunned down in the Russian capital on Putin's birthday in 2006.
"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 viewing the scene.
Washington was among the first to condemn the killing.
"We call upon the Russian government to conduct a prompt, impartial, and transparent investigation ... and ensure that those responsible for this vicious killing are brought to justice," Obama said in a statement.
Human Rights Watch also urged the Russian authorities to "thoroughly and impartially" investigate the murder.
Kasyanov said organisers had scrapped plans to hold the rally and planned to conduct a funeral march in central Moscow instead.
Boris Akunin, one of Russia's best-loved authors, said Nemtsov was one of the most fearless people he had ever met.
Cursing the killers he wrote on Facebook: "And you be damned, ghouls."
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