More than 20,000 protesters have marched through Moscow to protest against the killing of opposition leader Boris Nemtsov. Crowds of mourners and activists were seen waving flags and holding placards of the murdered Russian politician's face as they made their way through the city centre towards where he was killed on Friday night. As the vast protest snaked its way through the Russian capital, men, women and children could be seen holding back tears as they held up signs reading: 'I am not afraid.' Meanwhile in London, dozens of protesters waving Ukrainian flags gathered outside the Russian Embassy to demonstrate against Vladimir Putin's government. Scroll down for video . More than 20,000 protesters marched through Moscow in a mass demonstration against the killing of Boris Nemtsov, organisers said . Thousands of protesters marched in Moscow in memory of Boris Nemtsov, who was shot dead in a 'contract hit' on Friday . The huge crowds marched behind a huge banner reading: 'Those bullets for everyone of us, heroes never die!' The demonstrators waved hundreds of Russian flags as they marched along streets next to the river in the Russian capital . Thousands of protesters have marched through Moscow following the killing of opposition leader Boris Nemtsov . Crowds of mourners and activists were seen waving flags and holding placards showing the Russian opposition leader's face . Mourners make their way through the centre of the Russian capital and towards where Mr Nemtsov was killed . Dozens of protesters with Ukrainian flags also gathered outside the Russian Embassy in London to voice their anger at Mr Nemtsov's death . Marina Litvinenko, the wife of murdered Russian spy Alexander Litvinenko, joined demonstrators outside the embassy in London . There were an estimated 20,000 on the streets, while organisers claimed there were as many as 70,000. Demonstrations on this scale are rarely seen in Russia, where the government does not often give approval to mass protests. Hundreds had already planned to rally against Vladimir Putin's today, but this protest was abandoned to instead demonstrate against Mr Nemtsov's death in an apparent 'contract hit' in the shadow of the Kremlin. Russian riot police, armed with batons, were observing the protest which remained peaceful. The mood was sombre, with heavy security as police helicopters flew overhead and police boats patrolled the river. Families, the old and young walked slowly, with many carrying portraits of Mr Nemtsov, who was shot dead while walking home from a restaurant in central Moscow on Friday night. '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 mass civil conflict,' Gennady Gudkov, an opposition leader, said before the march. 'The authorities are corrupt and don't allow any threats to them to emerge. Boris was uncomfortable for them.' The march follows last night's demonstration on the bridge where Mr Nemtsov was shot, with scores of mourners gathering to lay flowers. Among placards left among the tributes were some reading 'je suis Boris', 'Putin killed my friend' and 'we are all Nemtsov'. 'Boris Nemtsov - we will never forget, we will never forget': Tens of thousands gathered in Russia's capital for the march . Demonstrations on this scale are rarely seen in Russia, where the government does not often give approval to mass protests . Riot police lined the parade route but did not intervene as the protesters made their way through the city . A man holds a Ukrainian flag at the rally in memory of Mr Nemtsov, which was originally due to be a protest against the Kremlin but was abandoned to instead demonstrate against the opposition leader's death . A woman lays flowers at the place where Mr Nemtsov was gunned down on Friday night as he crossed the Great Moskvoretsky Bridge . On patrol: Russian riot police were observing the protest, attended by thousands which has so far remained peaceful . The vast crowd of thousands of Russians were marching to commemorate murder opposition leader Mr Nemtsov . Among placards left among the mourning tributes were some reading 'je suis Boris', 'Putin killed my friend' and 'we are all Nemtsov' Mourners: A woman holds back tears as the march makes it way through the streets of Moscow this afternoon . Protesters hold up portraits of Mr Nemtsov as they gathered in a square in the Russian capital to voice their anger . A man prepares portraits of Mr Nemtsov, with the opposition claiming the killing was politically motivated . Critics linked his killing to the deaths of journalists at the French political magazine, Charlie Hebdo, who were slaughtered by Islamic extremists after publishing cartoons of the Prophet Mohammed. CCTV footage showed the Russian opposition leader walking along the Great Moskvoretsky Bridge in Moscow, before a person is seen approaching him from the shadows. The gunman then makes a quick escape in a car nearby. In the days before his death, Mr Nemtsov, a former deputy prime minister, said he feared being killed by the Russian leader. 'I'm afraid Putin will kill me. I believe that he was the one who unleashed the war in the Ukraine. I couldn't dislike him more,' he said in an interview with Russia's Sobesednik news website. He also revealed his 86-year-old mother's fears that he was be assassinated because of his outspoken views. '"When will you stop cursing Putin? He'll kill you for that." She was completely serious,' he added. Opposition activists also blamed the Russian President for his death, claiming they were in 'no doubt' it was politically motivated. Murdered spy Alexander Litvinenko's widow has said Boris Nemtsov's shooting was the Russian government's way of saying that anyone who spoke out against Vladimir Putin would be killed. Thousands sported Russian flags as they marched across a bridge, chanting anti-Kremlin slogans . In the days before his death, Mr Nemtsov, a former deputy prime minister, said he feared being killed by Russian President Vladimir Putin . A group of demonstrators rally outside St Basil's Cathedral in Moscow holding flags and banners showing the opposition leader . Two women hold signs reading 'These bullets in each of us' and 'Heroes do not die' as others carry posters saying 'I am not afraid' and 'No words' A grieving woman holds a photograph of the murdered Russian, who said he feared for his life just days before he was shot . Flowers were also laid outside the Russian Embassy in London where dozens gathered to commemorate the fallen politician . Former prime minister of Ukraine Yuliya Tymoshenko crosses herself at a memorial for Mr Nemtsov during a ceremony at Independence Square in Kiev . Footage emerged last night showing the killing, with a person jumping into a car and driving off . Mr Nemtsov was shot four times on a Moscow bridge after taking his Ukrainian model girlfriend Anna Duritskaya to dinner . The father-of-four was shot four times by assailants previously reported as having been in a white car as he walked across a bridge over the Moskva River . Marina Litvinenko described news of the veteran liberal politician's shooting as 'absolutely devastating'. She was asked on BBC Radio 4's Broadcasting House programme whether she believed the shooting of Mr Nemtsov near the Kremlin in Moscow was linked to her husband's poisoning. She replied: 'It's a different way to kill a person, but a way to present that anybody who will try to say something against us will be killed.' Ilya Yashin, a friend and fellow opposition leader, said he hoped the killing would not frighten people. 'Essentially it is an act of terror. It is a political murder aimed at frightening the population, or the part of the population that supported Nemtsov and did not agree with the government. I hope we won't get scared, that we will continue what Boris was doing. 'It was President Putin who created this atmosphere of hate in our country, the atmosphere of intolerance, which one way or another materialized in the bullet that killed my friend Boris Nemtsov.' Meanwhile the Kremlin, which uncharacteristically allowed the protest to go ahead, is investigating whether the killing is linked to the conflict in Ukraine. National investigators who answer to the Russian leader say they are pursuing several lines of inquiry, including the possibility that Mr Nemtsov, a Jew, was killed by radical Islamists or that the opposition killed him to blacken Putin's name. Putin's opponents say such suggestions show the cynicism of Russia's leaders as they whip up nationalism, hatred and anti-Western hysteria to rally support for his policies on Ukraine and deflect blame for an economic crisis. 'It is a blow to Russia. If political views are punished this way, then this country simply has no future,' Sergei Mitrokhin, an opposition leader, said of Mr Nemtsov's murder. Russian opposition leader Boris Nemtsov was shot dead in Moscow at the age of 55. His 87-year-old mother Dina had a premonition that her son would be killed, according to the politician. He told earlier this month how his mother warned him: 'When will you stop cursing Putin? He'll kill you for that.' Nemtsov studied physics at State University of Gorky and earned a PhD in Physics and Mathematics. In the wake of the Chernobyl disaster, Nemtsov organised a protest movement in his hometown, which prevented the construction of a new nuclear power plant in the region. The liberal reformer rose to prominence under Boris Yeltsin and became a fierce critic of Vladimir Putin. Nemtsov first gained an international profile after being spotted by former British premier Margaret Thatcher as a future leader of Russia. She praised his market reforms after visiting Nizhny Novgorod where as governor in the early 1990s he led spearheaded reforms. The father-of-four, 55, was Deputy Prime Minister of Russia from 1997 to 1998 during Boris Yeltsin's presidency. He was sentenced to 15 days in jail in January 2011 after being arrested at a New Year's Eve protest rally for 'disobedience towards police'. The politician founded a number of opposition movements after leaving the Russian parliament in 2003 and he had served as the co-chair of the opposition Republican Party of Russia - People's Freedom Party since 2012. He was a prominent and vocal critic of Mr Putin and wrote a number of reports in recent years linking Putin and his inner circle to corruption. It has been reported that Nemtsov angered Putin's government two years ago when he charged that billions of dollars had been stolen from funds designated for the 2014 Winter Olympics in Sochi, his hometown. He has written more than 60 academic publications about quantum physics, thermodynamics and acoustics and  designed n of antennas for space apparatuses. Jewish advocacy website AJC named Nemtsov as one of the most prominent Jews in Russia thanks to his mother's heritage. In his 1997 memoir, The Provincial Man, Nemtsov revealed that he was baptised Russian Orthodox in secret. He leaves behind his wife Raisa Akhmetovna and four children.
More than 20,000 march through Moscow to protest against the killing of Russian opposition leader Boris Nemtsov . Vast crowds of mourners and activists waved flags and held placards which read: 'I am not afraid'
Protest comes two days after Mr Nemtsov was shot dead in 'contract hit' as he walked along bridge near the Kremlin . CCTV footage shows person approaching Mr Nemtsov, shooting him and escaping in a nearby getaway car . Widow of murdered Russian spy Alexander Litvinenko says she had 'no doubt' killing was politically motivated . Meanwhile dozens gather outside the Russian Embassy in London to protests against Putin government .