Boris Nemtsov, a fiery critic of Russian president Vladimir Putin who once said Putin wanted him dead, was shot to death Friday in central Moscow near Red Square.
Nemtsov was hit four times by bullets fired by an unidentified man in a car. At least seven shots were fired at him from a passing car, reported USA Today.
Russian-language news website Meduza, however, reported that several people got out of a car and shot Nemtsov as he was walking along the Bolshoy Zamoskvoretsky Bridge.
Nemtsov's death "appears to be a contract killing", said Russian news agency, Interfax, quoting unidentified sources.
The Ministry of the Interior of the Russian Federation said Moscow Police were attempting to detain the killers.
Media reports in the West said Russian state TV was reporting over and over that Nemtsov was murdered while walking "with a young woman who was born in Kiev," the capital of Ukraine.
Nemtsov, 55, was a leading Russian opposition politician, former Deputy Prime Minister and leading economic reformers in the 1990s. He later fell out of favor with Putin and became one of the Russian president's most outspoken critics.
Nemtsov was also an outspoken critic of Putin, who he repeatedly attacked for igniting the war in Ukraine that has taken over 5,000 lives and threatens to ignite a war with NATO.
In an interview earlier this month, Nemtsov said he feared Putin would have him killed because of his opposition to the war in Ukraine, said the BBC.
"I'm afraid Putin will kill me," he said in a story written in Russian and posted on the Sobesednik news website February 10.
"I believe that he was the one who unleashed the war in the Ukraine," he said. "I couldn't dislike him more."
Thorbjorn Jagland, secretary general of the Council of Europe, condemned Nemtsov's killing.
"I am shocked and appalled key opposition leader Boris Nemtsov was shot. Killers must be brought to justice", he said in a tweet.
In his last tweet, Nemtsov sent out an appeal for Russia's divided opposition to unite at an anti-war march he was planning for Sunday, March 1.
"If you support stopping Russia's war with Ukraine, if you support stopping Putin's aggression, come to the Spring March in Maryino on 1 March," he tweeted.
Putin condemned Nemtsov's killing in a short statement, said a Kremlin spokesman.