Boris Nemtsov was one of Russia's leading economic reformers in the 1990s (file photo from 2009)
A leading Russian opposition politician, former Deputy Prime Minister Boris Nemtsov, has been shot dead in Moscow, Russian officials say.
An unidentified attacker shot Mr Nemtsov four times in central Moscow, a source in the law enforcement bodies told Russia's Interfax news agency.
He was reportedly shot near the Kremlin while walking with a woman.
He died just before a march in Moscow against the war in Ukraine which he was actively promoting.
In a recent interview, he had said he feared Russian President Vladimir Putin would have him killed because of his opposition to the war in Ukraine.
Mr Putin has condemned the killing, a Kremlin spokesman told news agencies.
Mr Nemtsov, 55, served as first deputy prime minister under the late 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, Mr Nemtsov became an outspoken opposition politician.
Thorbjorn Jagland, secretary general of the Council of Europe, condemned the killing, saying in a tweet: "I am shocked and appalled key opposition leader Boris Nemtsov was shot. Killers must be brought to justice."
According to Russian-language news website Meduza, "several people" got out of a car and shot him.
He was shot in the back with a pistol from a white car which fled the scene, Interfax's source said.
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.
"At the Bolshoy Zamoskvoretsky Bridge. I see the body and lots of police around it."
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 on 10 February (in Russian).
I believe that he was the one who unleashed the war in the Ukraine," he said. "I couldn't dislike him more."
Mr Putin has been widely accused of fomenting the bloody rebellion in east Ukraine - an accusation he denies.
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".