Medics stand near the body of Boris Nemtsov, who was shot dead, in central Moscow February 28, 2015. Nemtsov, a Russian opposition politician and former deputy prime minister who was an outspoken critic of President Vladimir Putin, was shot dead metres from the Kremlin in central Moscow late on Friday. [Photo/Agencies]

MOSCOW - Boris Nemtsov, a Russian opposition politician and former deputy prime minister who was an outspoken critic of President Vladimir Putin, was shot dead metres from the Kremlin in central Moscow late on Friday.

Nemtsov, 55, was shot four times in the back, the Interior Ministry said. A police spokeswoman on the scene said he had been walking on a bridge over the Moskva River with a Ukrainian woman.

Putin condemned the killing and took the investigation under presidential command, saying it could have been a contract killing and a "provocation" on the eve of a big opposition protest that Nemtsov had been due to lead in Moscow on Sunday.

Police cars sealed off the bridge close to the red walls of the Kremlin and Red Square, and an ambulance was on the scene.

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

A police spokesman on the scene said Nemtsov had been shot at from a passing white car that fled the scene. The woman was being interviewed by police.

Mikhail Kasyanov, a fellow opposition leader, told reporters at the bridge: "That a leader of the opposition could be shot beside the walls of the Kremlin is beyond imagination. There can be only one version: that he was shot for telling the truth."

Kasyanov, a former prime minister under Putin, called Nemtsov a "fighter for the truth".