Several gunmen shot and killed Russian opposition leader Boris Nemtsov Friday outside the Kremlin in Moscow. 

Nemtsov, 55, was walking across the bridge in view of the Kremlin with a female companion when unknown attackers in a white car shot him four times in the back, BBC News reported. Hours earlier, Nemtsov appealed to the Russian people for support for a march Sunday in Moscow against the war in Ukraine. Nemtsov had said that he feared President Putin would kill him because of his opposition to the Ukrainian war. 

President Putin condemned the murder and seized control of the invesitgation, Putin's spokesman Dimtry Peskov said, according to USA Today. Peskov added that the murder could be connected to the opposition march. The Russian Interior ministry has tried to reassure the public that Moscow police were working to catch the killers. They have surveillance footage that identifies the license plates of the vehicle the killers escaped in. 

Nemtsov's female companion, a woman from Ukraine's capital Kiev, was not injured during the attacks. The assassination occurred around 11:40 p.m. Friday on the Bolshoy Kamenny Bridge, BBC News said. 

Nemtsov served as first deputy prime minister under President Boris Yeltsin in the 1990s, and became known as an economic reformer while serving as governor of Nizhny Novgorod. He has been outspoken in his disdain for President Putin, the war in Ukraine and Russia's involvement. 

According to his lawyer, Nemtsov received death threats on social media in recent months, and authorities speculate that he was targeted for his opposition to the Russian government and its involvement in Ukraine. 

However, Russian opposition figure Mikhail Kasyanov, a former Russian prime minister, said the protest march Sunday would proceed as planned, USA Today reported.