Boris Nemtsov, a prominent Russian opposition politician and former deputy prime minister, has been shot dead in Moscow -- two days before a planned protest against President Vladimir Putin's government.
Russia's Interior Ministry has confirmed that Nemtsov was killed on Bolshoi Moskvoretsky Bridge close to the Kremlin late on the evening of February 27.
The slaying adds to a growing list of dead Russian activists, journalists, and politicians who had challenged the Kremlin since the collapse of the Soviet Union.
Nemtsov, 55, was a sharp critic of Putin, assailing the government's inefficiency, rampant corruption and the Kremlin's policy on Ukraine.
In the run-up to last year's Sochi Winter Olympics, Nemtsov authored reports alleging massive corruption in preparations for the games.
He was reportedly working on a document containing evidence that he said proved Russia's direct involvement in the separatist rebellion in eastern Ukraine.
Ukraine and the West have accused Putin of being behind the war in eastern Ukraine -- an accusation the Russian leader denies.
Nemtsov's lawyer told RFE/RL's Russian Service that he was a victim of Russia's political regime, no matter who pulled the trigger.
In a recent interview, Nemtsov had voiced fears that Putin would have him killed because of his opposition to the war in Ukraine.
Interior Ministry spokeswoman Yelena Alexeyeva told the media that Nemtsov was walking with a Ukrainian female acquaintance when a vehicle drove up and unidentified assailants opened fire, killing him with four bullets to the back. The woman wasn't hurt.
Pictures circulated on social networks showed several bouquets flowers had been placed at the scene of the crime.
The brazen killing just outside the Kremlin recalled the high-profile assassination of anti-Putin journalist Anna Politkovskaya, who was shot dead in Moscow on Putin's birthday in 2006.
Putin himself was quick to condemn Nemtsov's killing.
According to his spokesman, Dmitry Peskov, Putin said the "cruel murder" had "all the signs of a contract killing" and called it a provocation -- a word meant to suggest Nemtsov was killed in order to blacken the government's reputation.
In reaction to Peskov's statement, chess champion and fellow Putin critic Garry Kasparov tweeted: '[Journalist Anna] Politkovskaya was gunned down. [Malaysian Airlines flight] MH17 was shot out of the sky [over the conflict zone in eastern Ukraine]. Now Boris is dead. As always, Kremlin will blame opposition, or CIA, whatever.'
Nemtsov's death comes just two days before a planned protest against Putin's rule, which organizers hope will be one of the largest in months.
Peskov said Putin had expressed his condolences and ordered the chiefs of the Investigative Committee, the Interior Ministry and the Federal Security Service to form an investigative group and keep the probe under their personal control.
U.S. President Barack Obama said in a statement that "the United States condemns the brutal murder of Boris Nemtsov."
The White House statement also called on the Russian government to conduct a "prompt, impartial and transparent investigation" and to "ensure those responsible are brought to justice."
The statement offered sincere condolences to Nemtsov's family, and "to the Russian people, who have lost one of the most dedicated and eloquent defenders of their rights."
U.S. Secretary of State John Kerry said in a separate statement, that Nemtsov "committed his life to a more democratic, prosperous, open Russia."
Kerry said Nemtsov "sought to reform and open Russia, and to empower the Russian people to have a greater say in the life of their country."
Ukrainian President Petro Poroshenko called Nemtsov a personal friend and a "bridge" between the two countries.
Poroshenko wrote on Twitter: "Shock. They killed Boris. It's hard to believe. I have no doubt the killers will be found. Sooner or later. Eternal memory..."
Speaking to RFE/RL's Russian Service, Nemtsov's lawyer Vadim Prokhorov said Nemtsov had received threats many times, mainly through social networks. "Whoever pulled the trigger, Boris Nemtsov was killed by the political regime," Prokhorov said.
"It is necessary to work out who killed Nemtsov, but these kinds of murders are going to happen if we hear words from the upper echelons that those who want to see Russia in Europe are national traitors," he added.
Dmitry Gudkov, an opposition lawmaker with the Just Russia party, wrote on Twitter, "This is clearly a political killing. If it wasn't ordered, it was the result of the hatred propagandized by our authorities."
Once considered a potential successor to Russia’s first post-Soviet president, Boris Yeltsin, Nemstov became an opposition politician and fierce critic of Russian President Vladimir Putin.
After working as a research scientist in the last years of the Soviet era, Nemtsov rose to prominence as governor of the Nizhny Novgorod region in central Russia and became a vice prime minister in the late 1990s during Boris Yeltsin's presidency.
Nemtsov left the Duma, Russia’s Parliament, in 2003, and founded and led several opposition parties and groups, the latest being the Republican Party of Russia -- People’s Freedom Party.
An accomplished orator, Nemtsov was one of the key speakers at mass opposition rallies against Putin's return to the Kremlin in 2012.
He also authored critical reports about corruption under Putin.
In 2013, he said up to $30 billion of the estimated $50 billion assigned to the Winter Olympics Russia was to host in Sochi had gone missing.
The Kremlin has denied the claims.
Exiled former Russian tycoon Mikhail Khodorkovsky, himself a Putin opponent, said his family was grieving. "We all loved him. With a devil-may-care attitude but a very good guy," Khodorkovsy wrote on Twitter.
Vygaudas Usackas, the head of the European Union's delegation in Russia said he was shocked by Nemtsov's murder and urged Russian authorities to "swiftly" find those responsible and bring them to justice.
Former U.S. Ambassador to Russia Michael McFaul said in a tweet: 'Nemtsov was a real patriot, who believed in the possibility of Russia's greatness. I cry now both for his family & the country he so loved.'
Thorbjorn Jagland, Secretary-General of the Council of Europe, also reacted to the murder on twitter: "I am shocked and appalled key opposition leader Boris Nemtsov was shot. Killers must be brought to justice."
Human Rights Watch also urged the Russian authorities to "thoroughly and impartially" investigate the killing.
Nemtsov told Russia's Sobesednik news website earlier this month that he feared for his own life.
"I'm afraid Putin will kill me," he said on February 10.
Nemtsov said of Putin, "I believe that he was the one who unleashed the war in the Ukraine. I couldn't dislike him more."
And in a blog posted on the liberal radio station Echo Moskvy just hours before he was gunned down, Nemtsov urged Muscovites to attend the opposition rally "against Putin's aggression" in Ukraine on March 1.
Former Russian Prime Minister Mikhail Kasyanov, now also in opposition, said that the rally organizers decided that instead of the planned demonstration on March 1 on the capital's southeastern outskirts they will stage a protest in the center of Moscow to commemorate Nemtsov.
At the site of the murder, Kasyanov said, "In the 21st century, a leader of the opposition is being demonstratively shot just outside the walls of the Kremlin!"
He concluded, "The country is rolling into the abyss."
With reporting by Reuters, AP, AFP, Interfax, Itar-Tass