Boris Nemtsov, the former Russian deputy prime minister and fierce critic of Vladimir Putin, has died after being shot four times by a passing car in Moscow, officials have confirmed.
The 55-year-old was targeted by an unidentified attacker near the Kremlin during the early hours of Saturday morning - and was shot in the back.
According to colleagues, he was working on a report which apparently included concrete evidence that Russia was directly involved in the separatist movement which erupted in Ukraine last year.
At the time of the attack, Mr Nemtsov was walking along a bridge with a Ukrainian woman, who was uninjured in the shooting. She is now being questioned by police.
The politician had received death threats on social media, but no steps were taken by the police to offer him protection, according to his lawyer, Vadim Prokhorov.
A senior opposition politician, Mr Nemtsov was an outspoken detractor of Vladimir Putin's administration - and had planned to attend an "anti-crisis march" through the capital on Sunday.
The economist had said in an interview with Russian media: "Every time I called (my mother), she laments: 'When are you going to stop scolding Putin? He will kill you!'"
Mr Putin has condemned the death, which he said looked like a "contract killing" - adding that he will personally oversee the investigation into the shooting.
Meanwhile, the White House has demanded that the investigation is "prompt, impartial and transparent".
President Obama, who met Mr Nemtsov in 2009, described him as a "tireless advocate" for the rights of Russian citizens, and cited his work in fighting corruption.
In a statement, Britain's Foreign and Commonwealth Office said it was "shocked and saddened" by the news.
"We deplore this criminal act. Those responsible must be brought to justice," it added.
Garry Kasparov, chairman of the Human Rights Foundation, tweeted: "Devastated to hear of the brutal murder of my long-time opposition colleague Boris Nemtsov. Shot four times, once for each child he leaves."
Widely regarded as the greatest chess player of all time, the political activist added: "Politkovskaya was gunned down. MH17 was shot out of the sky. Now Boris is dead. As always, Kremlin will blame opposition, or CIA, whatever."
According to the Financial Times, Mr Nemtsov had wanted to tackle the stagnant wages and soaring inflation that has blighted Russia's economy in recent times - with the rouble collapsing and Western nations imposing sanctions over the government's involvement in the Ukraine crisis.
"They believe that the embargo on imported foods is America's fault, and they were surprised when I told them no, that was not Obama, it was Putin. This is what we need to make people aware of: the crisis, that's Putin," Mr Nemtsov said.
