Russian President Vladimir Putin pledged Saturday to do everything to bring to justice those responsible for the death of Russian opposition leader Boris Nemtsov who was shot dead at about midnight in downtown Moscow.

"We will do everything to ensure that the perpetrators of this foul and cynical crime and those who stand behind them are properly punished," Putin said in a message of condolences to Nemtsov's mother, Dina Eidman.

"Boris Nemtsov has left his mark in the history of Russia, in its political and public life. He occupied significant posts in a difficult time of transition in this country. He always openly and honestly voiced and upheld his views," the president added.

The Russian Investigative Committee has launched a comprehensive investigation into the incident, which are pursuing several lines, including the possibility of an attempt to destabilize the political situation in Russia, the internal situation in Ukraine and possible connections with Nemtsov's business activities or personal animosity.

"For now, the investigation is analyzing several theories, including murder as an act of provocation to destabilize the political situation in the country, and Nemtsov's personality could have been a kind of sacrifice for those who stop at nothing to attain their political ends," Vladimir Markin, spokesman of the committee, was quoted as saying by the Interfax news agency.

Noting that Nemtsov had reportedly received threats due to his position on the killing of Charlie Hebdo journalists in Paris in January, Markin said the possibility of Islamic extremist involvement is also being analyzed.

Earlier in the day, Russian Prime Minister Dmitry Medvedev said he was shocked by the assault, adding that "this is a personal tragedy for his relatives and friends and a great loss for our society."

On Saturday afternoon, crowds of people, some with flowers in their hands, flocked to the site of the attack to mourn the death of the former Russian Deputy Prime Minister.

According to preliminary investigation, Nemtsov was hit by four shots and six shell casings have been found at the site.