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.
Ukraine crisis: A timeline of the conflict
Ukraine crisis: A timeline of the conflict
1/22 30 November 2013
Public support grows for the “Euromaidan” anti-government protesters in Kiev demonstrating against Yanukovych’s refusal to sign the EU Association Agreement as images of them injured by police crackdown spread.
2/22 20 February 2014
Kiev sees its worst day of violence for almost 70 years as at least 88 people are killed in 48 hours, with uniformed snipers shooting at protesters from rooftops.
3/22 22 February 2014
Yanukovych flees the country after protest leaders and politicians agree to form a new government and hold elections. The imprisoned former Prime Minister, Yulia Tymoshenko, is freed from prison and protesters take control of Presidential administration buildings, including Mr Yanukovych's residence.
Genya Savilov/AFP/Getty Imageses
4/22 27 February 2014
Pro-Russian militias seize government buildings in Crimea and the new Ukrainian government vows to prevent the country breaking up as the Crimean Parliament sets a referendum on secession from Ukraine in May.
Sean Gallup/Getty Images
5/22 16 March 2014
Crimea votes overwhelmingly to secede from Ukraine and join Russia in a ballot condemned by the US and Europe as illegal. Russian troops had moved into the peninsula weeks before after pro-Russian separatists occupied buildings.
6/22 6 April 2014
Pro-Russian rebels seize government buildings in the eastern cities of Donetsk, Luhansk and Kharkiv, calling for a referendum on independence and claiming independent republic. Ukraine authorities regain control of Kharkiv buildings on 8 April after launching an “anti-terror operation” but the rest remain out of their control.
7/22 7 June 2014
Petro Poroshenko is sworn in as Ukraine's president, calling on separatists to lay down their arms and end the fighting and later orders the creation of humanitarian corridors, since violated, to allow civilians to flee war zones.
8/22 27 June 2014
The EU signs an association agreement with Ukraine, along with Georgia and Moldova, eight months after protests over the abandonment of the deal sparked the crisis.
LOUISA GOULIAMAKI/AFP/Getty Images
9/22 17 July 2014
Malaysian Airlines flight MH17 is shot down over eastern Ukraine, killing all 298 people on board. Ukrainian intelligence officials claim it was hit by rebels using a Buk surface-to-air launcher in an apparent accident.
10/22 22 August 2014
A Russian aid convoy of more than 100 lorries enters eastern Ukraine and makes drop in rebel-controlled Luhansk without Government permission, sparking allegations of a “direct violation of international law”.
11/22 29 August 2014
Nato releases satellite images appearing to show Russian soldiers, artillery and armoured vehicles engaged in military operations in eastern Ukraine.
12/22 8 September 2014
Russia warns that it could block flights through its airspace if the EU goes ahead with new sanctions over the ongoing crisis and conflict
13/22 17 September 2014
Despite the cease-fire and a law passed by the Ukrainian parliament on Tuesday granting greater autonomy to rebel-held parts of the east, civilian casualties continued to rise, adding to the estimated 3,000 people killed
14/22 16 November 2014
The fragile ceasefire gives way to an increased wave of military activity as artillery fire continues to rock the eastern Ukraine's pro-Russian rebel bastion of Donetsk
15/22 26 December 2014
A new round of ceasefire talks, scheduled on neutral ground in the Belariusian capital Minsk, are called off
16/22 12 January 2015
Soldiers in Debaltseve were forced to prepare heavy defences around the city; despite a brief respite to the fighting in eastern Ukraine, hostilities in Donetsk resumed at a level not seen since September 2014
17/22 21 January 2015
13 people are killed during shelling of bus in the rebel-held city of Donetsk
18/22 24 January 2015
Ten people were killed after pro-Russian separatists bombarded the east Ukrainian port city of Mariupol
19/22 2 February 2015
There was a dangerous shift in tempo as rebels bolstered troop numbers against government forces
20/22 11 February 2015
European leaders meet in Minsk and agree on a ceasefire in eastern Ukraine beginning on February 14. From left to right: Belarus President Alexander Lukashenko, Russian President Vladimir Putin, German Chancellor Angela Merkel, France's President Francois Hollande and Ukrainian President Petro Poroshenko.
MAXIM MALINOVSKY | AFP | Getty Images
21/22 13 February 2015
Pro-Russian rebels in the city of Gorlivka, in the Donetsk region, fire missiles at Ukrainian forces in Debaltseve. Fighting continued in Debaltseve for a number of days after the Minsk ceasefire began.
ANDREY BORODULIN | AFP | Getty Images
22/22 18 February 2015
Ukrainian soldiers repair the bullet-shattered windshield of their truck as their withdraw from the strategic town of Debaltseve. Following intense shelling from pro-Russian rebels, Ukrainian forces began to leave the town in the early hours of February 18.
Brendan Hoffman | Getty Images
"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