The Russian opposition politician and former deputy prime minister Boris Nemtsov was shot and killed by four shots in central Moscow late on Friday, the Russian government said in a statement, according to the Interfax news agency.Known as an outspoken critic of President Vladimir Putin, Nemtsov, 55, was shot four times in the back, the Interior Ministry said. A police spokesman on the scene said Nemtsov had been shot from a passing white car that fled the scene.Putin condemned the killing and said it could have been a "provocation" on the eve of a big opposition protest, which Nemtsov had been due to lead in Moscow on Sunday. President Putin has assumed "personal control" of the investigation into the killing, said his spokesman Dmitry Peskov.Mikhail Kasyanov, a fellow opposition leader, told reporters: "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."In a recent interview, Mr Nemtsov had said he feared Mr Putin would have him killed because of his opposition to the war in Ukraine.There was an immediate outpouring of shock and grief on social media while thousands of people paid their respects at the scene of the murder from early on Saturday morning, laying flowers and lighting candles. A piece of white paper saying "We are all Nemtsov" stood among the flowers.Nemtsov was a deputy prime minister under President Boris Yeltsin in the 1990s, and an outspoken critic of Yeltsin's successor Vladimir Putin.Nemtsov was also one of the leaders of mass rallies in the winter of 2011-12 that became the biggest protest against Putin since the former KGB spy rose to power in 2000.After leaving parliament in 2003, he helped establish and led several opposition parties and groups.