Russian opposition politician Boris Nemtsov has been shot dead in Moscow.

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 capital.

The Interior Ministry, which oversees Russia"s police force, said that Nemtsov was shot four times from a passing car as he was walking on a bridge just outside the Kremlin.

Nemtsov served as deputy prime minister in the 1990s and later became a prominent opponent to Putin, helping organise street protests and exposing official corruption.

Police cars blocked the street where he was shot. An ambulance was also nearby.

"Nemtsov BE died at 2340 hours as a result of four shots in the back," an Interior Ministry spokeswoman said.