A prominent Russian opposition leader has been shot multiple times and killed while walking on a bridge in central Moscow, investigators said Friday. Boris Nemtsov, 55, was shot from a passing car on a bridge just outside the Kremlin, the Russian investigators said in a statement. An unknown attacker fired between 7 and 8 shots on the politician, who was walking over the Big Moskvoretsky bridge, Vladimir Markin, spokesman for the Investigative Committee, said. Nemtsov, who served as a deputy prime minister under Boris Yeltsin, had been a constant critic of President Vladimir Putin since he came to power in 2000. Since 2013 Nemtsov was a member of the parliament of Yaroslavl, a region north of Moscow. The killing came less than two days before a big opposition protest has been called for Moscow and other Russian cities. Nemtsov and his People's Freedom Party had been co-sponsors of the main march scheduled to take place Sunday in a Moscow suburb.