Updated 5.59pm: Russian opposition leader Boris Nemtsov, a fierce critic of President Vladimir Putin, has been shot dead in central Moscow before a major opposition rally this weekend.

"In central Moscow, a man with documents in the name of Boris Yefimovich Nemtsov was killed," an interior ministry spokesman told Agence France-Presse, declining to give further details.

Russia's Investigative Committee confirmed the death, saying it had opened a criminal probe.

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

The committee, which reports directly to Putin, said that "experienced" investigators had been put on the case.

Nemtsov, 55, 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 helped establish and led several opposition parties and groups.

Nemtov's murder comes ahead of a major opposition rally scheduled to take place on March 1.

US President Barack Obama has condemned what he says is the brutal and vicious murder of Russian opposition leader Boris Nemtsov.

Mr Obama has called on the Russian government to make sure the probe is prompt, impartial, and transparent.

He says he admired Mr Nemtsov's courageous dedication to fighting corruption and willingness to share candid views.