A Russian opposition leader, Boris Nemtsov, was shot dead by an unidentified attacker who fired multiple rounds at the politician as he was walking in central Moscow late Friday.

A Russian opposition leader, Boris Nemtsov, was shot dead by an unidentified attacker who fired multiple rounds at the politician as he was walking in central Moscow late Friday.

The murder of the 55-year-old politician, who has been an outspoken critic of Russia's involvement in Ukraine, came ahead of a major opposition rally this weekend.

US President Barack Obama condemned the "vicious murder" of Nemtsov, whom he had met on a visit to Moscow, and called on the Russian government to conduct a prompt and impartial investigation of the killing.

The Kremlin swiftly dubbed his death a provocation.

The murder "bears the hallmarks of a contract killing," Putin spokesman Dmitry Peskov said, adding it was a provocation.

"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.

The Investigative Committee, which reports directly to Putin, confirmed Nemtsov's death, saying it had opened a criminal probe.

The committee said "experienced" investigators had been put on the case.

Nemtsov launched his political career as the governor of 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.

Nemtsov's murder comes ahead of a major opposition rally scheduled to take place on Sunday.

Interior Minister Vladimir Kolokoltsev has taken charge of the criminal probe.