A leading Russian opposition politician, former Deputy Prime Minister Boris Nemtsov, has been shot dead in Moscow, Russian officials say.
An unidentified attacker in a car shot Mr Nemtsov four times in the back as he crossed a bridge in view of the Kremlin, police say.
He died hours after appealing for support for a march on Sunday in Moscow against the war in Ukraine.
Russian President Vladimir Putin has condemned the murder, the Kremlin says.
President Putin has assumed "personal control" of the investigation into the killing, said his spokesman Dmitry Peskov.
Investigators said the murder could have been "a provocation aimed at destabilising the country".
The investigative committee said in a statement that several motives for the killing were being considered including "Islamic extremism".
US President Barack Obama condemned the "brutal murder" and called on the Russian government to conduct a "prompt, impartial and transparent investigation".
Ukrainian President Petro Poroshenko described Mr Nemtsov as a "bridge between Ukraine and Russia".
"The murderers' shot has destroyed it. I think it is not by accident," he said in a statement published on his administration's Facebook page.
In a recent interview, Mr Nemtsov had said he feared Mr Putin would have him killed because of his opposition to the war in Ukraine.
Mr Nemtsov, 55, served as first deputy prime minister under President Boris Yeltsin in the 1990s.
He had earned a reputation as an economic reformer while governor of one of Russia's biggest cities, Nizhny Novgorod.
Falling out of favour with Yeltsin's successor, Mr Putin, he became an outspoken opposition politician.
Analysis: Sarah Rainsford, BBC Moscow correspondent
A lawyer for Mr Nemtsov reported that he had received death threats over social media in recent months; but for now there's only speculation as to why he was targeted. He openly opposed Moscow's role in the crisis in Ukraine - and the annexation by Russia of Crimea.
He had been planning a rare public protest on Sunday against both things - and a growing economic crisis in this country.
Since his death, social media has been flooded with tributes to a man remembered by friends as decent, honest and a democrat. He had been pushed to the political margins in Vladimir Putin's Russia, but he was still prominent enough for someone to want to kill him.
Mr Nemtsov was shot at around 23:40 (20:40 GMT) on Friday while crossing Bolshoy Moskvoretsky Bridge accompanied by a woman, Russia's interior ministry said.
He was shot with a pistol from a white car which fled the scene, a police source told Russia's Interfax news agency.
According to Russian-language news website Meduza, "several people" got out of a car and shot him.
One of the politician's colleagues in his RPR-Parnassus party, Ilya Yashin, confirmed Mr Nemtsov's death.
"Unfortunately I can see the corpse of Boris Nemtsov in front of me now," he was quoted as saying by Russia's lenta.ru news website.
Flowers were left at the site of the shooting through the night.
Violent deaths of Putin opponents
April 2003 - Liberal politician Sergey Yushenkov assassinated near his Moscow home
July 2003 - Investigative journalist Yuri Shchekochikhin died after 16-day mysterious illness
July 2004 - Forbes magazine Russian editor Paul Klebnikov shot from moving car on Moscow street, died later in hospital
October 2006 - Investigative journalist Anna Politkovskaya shot dead outside her Moscow apartment
November 2006 - Former Russian spy Alexander Litvinenko died nearly three weeks after drinking tea laced with polonium in London hotel
March 2013 - Boris Berezovsky, former Kremlin power broker turned Putin critic, found dead in his UK home
In his last tweet, Mr Nemtsov sent out an appeal for Russia's divided opposition to unite at an anti-war march he was planning for Sunday.
"If you support stopping Russia's war with Ukraine, if you support stopping Putin's aggression, come to the Spring March in Maryino on 1 March," he wrote.
Speaking earlier this month to Russia's Sobesednik news website, he had spoken of his fears for his own life.
"I'm afraid Putin will kill me," he said in the article (in Russian) on 10 February.
"I believe that he was the one who unleashed the war in the Ukraine," he added. "I couldn't dislike him more."
Mr Putin has been widely accused of fomenting the bloody rebellion in east Ukraine - an accusation he denies. Fighting there followed Russia's annexation of Crimea in March last year.
Almost 5,800 people have died and at least 1.25 million have fled their homes, according to the UN.
The Ukrainian government, Western leaders and Nato say there is clear evidence that Russia is helping the rebels with heavy weapons and soldiers.
Independent experts echo that accusation while Moscow denies it, insisting that any Russians serving with the rebels are "volunteers".