Russia March
People hold flags and posters during a march to commemorate Kremlin critic Boris Nemtsov, who was shot dead on Friday night, in central Moscow on 1 March 2015.Reuters

Russian politician Boris Nemtsov's murder has raised fears of a civil war breaking out in the country, according to opposition leaders and Putin critics.

Mikhail Khodorkovsky, a Russian dissident who lives in exile in Switzerland, said on his website that Russians were close to a 'precipice of all-out war' following the murder.

Opposition leader Gennady Gudkov also warned of a 'mass civil conflict'. "If we can stop the campaign of hate that's being directed at the opposition, then we have a chance to change Russia. If not, then we face the prospect of a mass civil conflict," Gudkov told Reuters.

Nemtsov was shot in his back four times last Friday when he was on a bridge close to Kremlin. Opposition leaders have blamed Putin for conspiring to kill the former deputy prime minister. "They've murdered Boris. A hundred meters from the Kremlin. Right in the heart of a city inundated with thousands of policemen and special services personnel," Khodorkovsky wrote in his blog.

"I know that for many people Boris's death will become so much of a Rubicon that the entire country may become different. But different in what way? Will we find ourselves standing even closer to the precipice of all-out war of everybody against everybody?" he added.

Thousands of protesters took to the streets of Moscow on Sunday to mourn the death of Nemtsov. "This is a march for all the people who have come to understand that we have reached a dangerous point," Gudkov told The Moscow Times.

Gudkov's son and opposition State Duma deputy Dmitry Gudkov has also reportedly received death threats with some messages on his social media accounts suggesting that he was 'next on the list'.