Nemtsov, a former Russian deputy prime minister and outspoken critic of President Vladimir Putin, died after being shot four times late on Friday near the Kremlin in central Moscow. Police cars blocked the street where he was shot.

"Nemtsov B.E. died at 2340 hours as a result of four shots in the back," a Russian Interior Ministry spokeswoman told Reuters news agency. A police spokesman on the scene said that 55-year-old Nemtsov was taking a walk over a bridge on the Moskva river with a Ukrainian woman when he was shot from a passing car.

Police were questioning the woman and had sealed off the bridge close to Kremlin and the Red Square.

Putin condemns assassination

Vladimir Putin condemned the murder, saying he would personally oversee the investigation.

Putin's spokesman Dmitry Peskov said the president expressed his condolences and ordered security agencies to investigate.

Peskov said the shooting could also have been a "provocation," considering that Nemtsov was planning to participate in a big protest organized by the opposition in Moscow on Sunday.

Ukrainian President Petro Poroshenko also condemned the killing and conveyed his condolences:

On the wrong side of Putin

Nemtsov began his political career as the governor of Nizhny Novgorod in central Russia and rose to the position of deputy prime minister in the 1990s under Boris Yeltsin, Russia's first democratically elected president.

Nemtsov became a part of the opposition after Putin came to power in 2000, but groups critical of Putin have largely been sidelined since he returned to the presidency after four years as prime minister under Dmitry Medvedev.

Police in Moscow have recently stepped up a crackdown on activists, jailing another opposition leader, Alexei Navalny, for 15 days, rendering him incapable of attending the event. Nemtsov's colleagues were also linking his murder to Putin's intolerance of dissent.

"This is payback for the fact that Boris consistently, for many many years, fought for Russia to be a free democratic country," Mikhail Kasyanov, former prime minister turned opposition leader, told journalists in Moscow.

mg/bk (Reuters, dpa, AFP, AP)