The Russian opposition politician shot dead in Moscow had spoken of his fear that Vladimir Putin would have him killed weeks before his murder.
Speaking to Russia's Sobesednik news website on 10 February, Boris Nemtsov said: "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."
Friends said he had received anonymous death threats over the internet.
"Boris periodically received anonymous threats on social networking sites...Boris was worried," said opposition politician Ilya Yashin.
"He said he was under threat but never wanted additional security. (He said) if they want to kill (me) they will kill (me).”
Mr Nemtsov was shot four times in the back by a gunman in a car as he crossed a bridge in view of the Kremlin just before midnight on Friday, police said.
The 55-year-old was walking with a Ukrainian woman after they ate at a restaurant in Red Square. She escaped unhurt.
The former Deputy Prime Minister, who fell out of favour under Putin’s regime, was due to attend an anti-government Spring March on Sunday protesting against Russia’s alleged backing of rebels in Ukraine.
The President condemned the killing, which his spokesperson Dmitry Peskov said could be a "provocation" coming just a day before the march Mr Nemtsov helped organise.
"It was mentioned by the President that it looks like a contract killing, while carrying all the signs of a provocation," Mr Peskov added.
The Kremlin said last night that President Putin will personally oversee the Interior Ministry's investigation into the murder.
International governments have expressed their condolences to Russia and called for a full and transparent inquiry.
A spokesperson for the Foreign and Commonwealth Office said: “We are shocked and saddened by news that former Russian Deputy Prime Minister Boris Nemtsov has been shot and killed in Moscow.
"Our thoughts are with his family and we offer our condolences to them. We deplore this criminal act.
"Those responsible must be brought to justice. We will continue to follow the situation closely."
Barack Obama said Mr Nemtsov was a ”tireless advocate“ for Russia and the rights of its citizens and praised him for fighting corruption.
The pair met in Moscow in 2009, the US President said, when the Russian was willing to ”share his candid views“.
"We offer our sincere condolences to Boris Efimovich (Nemtsov's) family and to the Russian people, who have lost one of the most dedicated and eloquent defenders of their rights," he added.
Hours before his death, Mr Nemtsov had been urging Russians to join his rally to defy the “mad, aggressive and deadly policy of war against Ukraine”.
"The country needs a political reform. When power is concentrated in the hands of one person and this person rules forever, this will lead to an absolute catastrophe, absolute," he was reported to have told Ekho Moskvy radio.
Ukrainian President Petro Poroshenko said he was shocked by the killing and hoped those responsible for removing a "bridge" between the two countries would be brought to justice.
Garry Kasparov, a pro-democracy activist and former world chess champion, said on his Facebook page: “Shot four times, once for each child he leaves behind. A man of Boris's quality no longer fit Putin's Russia.
"He always believed Russia could change from the inside and without violence; after 2012 I disagreed with this.
"When we argued, Boris would tell me I was too hasty and that in Russia you had to live a long time to see change. Now he'll never see it. Rest in peace."
As an economic reformer, Mr Nemtsov served as Deputy Prime Minister under the presidency of Boris Yeltsin in the 1990s, and enjoyed a successful spell as governor of a major city, but fell out of favour when Mr Putin took charge.
His death has focused attention on the treatment of Kremlin opponents in Mr Putin's third term, which has seen several leading critics have jailed or fleeing the country following mass rallies against the former KGB spy three years ago.
"That a leader of the opposition could be shot beside the walls of the Kremlin is beyond imagination. There can be only one version: that he was shot for telling the truth,” Mikhail Kasyanov, an opposition leader and a former Prime Minister under Putin, said at the scene.
There was no immediate claim of responsibility and former Soviet President Mikhail Gorbachev warned against jumping to conclusions.
"Certain forces will try to use the killing to their own advantage. They are thinking how to get rid of Putin," he said.
Another opposition figure, Ksenia Sobchak, said Mr Nemtsov had been preparing a report on the presence of Russian troops in Ukraine.
Nato and Western allies have presented what they claim is evidence that Russia has sent troops and weapons to back separatists fighting the government in the east but the Kremlin has continually denied the accusations.
Additional reporting by agencies
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