Russian police investigate the the body of Boris Nemtsov, a former Russian deputy prime minister and opposition leader at Red Square with St. Basil Cathidral in the background in Moscow, Russia, Saturday, Feb. 28, 2015.
Putin ordered Russia's top law enforcement chiefs to personally oversee the probe of Nemtsov's killing.
"Putin noted that this cruel murder has all the makings of a contract hit and is extremely provocative," presidential spokesman Dmitry Peskov said in remarks carried by Russian news agencies.
President Barack Obama called on Russia's government to perform a "prompt, impartial and transparent" investigation to bring the perpetrators to justice. Obama called Nemtsov a "tireless advocate" for the rights of Russian citizens.
Nemtsov assailed the government's inefficiency, rampant corruption and the Kremlin's Ukraine policy, which has strained relations between Russia and the West to a degree unseen since Cold War times.
Nemtsov said on radio just a few hours before his death that Putin plunged Russia into the crisis by his "mad, aggressive and deadly policy of war against Ukraine."
"The country needs a political reform," Nemtsov said, speaking on Ekho Moskvy radio. "When power is concentrated in the hands of one person and this person rules for ever, this will lead to an absolute catastrophe, absolute."
Ukrainian President Petro Poroshenko called Nemtsov a personal friend and a "bridge" between the two countries. He said on his Facebook that he hopes the killers will be punished.
Nemtsov's lawyer Vadim Prokhorov said the politician had received threats on social networks and told police about them, but authorities didn't take any steps to protect him.
The Russian Interior Ministry, which oversees Russia's police force, said that Nemtsov was killed by four shots in the back from a passing car as he was walking over a bridge just outside the Kremlin shortly after midnight.
Interior Ministry spokeswoman Yelena Alexeyeva told reporters that Nemtsov was walking with a female acquaintance, a Ukrainian citizen, when a vehicle drove up and unidentified assailants shot him dead. The woman wasn't hurt.
Mikhail Kasyanov, a former Russian prime minister now also in opposition, said he was shocked.
"In the 21st century, a leader of the opposition is being demonstratively shot just outside the walls of the Kremlin!" Kasyanov told reporters as Nemtsov's body placed in a plastic bag was removed on a rainy and cold night, as the Kremlin bells chimed nearby. "The country is rolling into the abyss."
Kasyanov said the rally organizers decided that instead of the planned demonstration on Moscow's southeastern outskirts they will stage a demonstration in the center of the capital to commemorate Nemtsov.
Garry Kasparov, a former chess champion who worked with Nemtsov to organize protests against Putin and now lives in the United States, tweeted: "Devastated to hear of the brutal murder of my long-time opposition colleague Boris Nemtsov. Shot 4 times, once for each child he leaves."
Opposition activist Ilya Yashin said on Ekho Moskvy that he last spoke with Nemtsov two days before the killing.
Yashin said he had no doubt that Nemtsov's murder was politically motivated.
"Boris Nemtsov was a stark opposition leader who criticized the most important state officials in our country, including President Vladimir Putin. As we have seen, such criticism in Russia is dangerous for one's life," he said.
Political analyst Stanislav Belkovsky told Ekho Mosvky that he did not believe that Nemtsov's death would in any way serve Putin's interests.
"But the atmosphere of hatred toward alternative thinkers that has formed over the past year, since the annexation of Crimea, may have played its role," Belkovsky said, referring to the surge of intense and officially endorsed nationalist discourse in Russia since it annexed Ukraine's Crimean Peninsula.
Irina Khakamada, a prominent opposition figure who co-founded a liberal party with Nemtsov, blamed a climate of intimidation and warned that the murder could herald a dangerous destabilization.
"It's a provocation that is clearly not in Putin's interests, it's aimed at rocking the situation," she said in remarks carried by RIA Novosti news agency.
Nemtsov served as a deputy prime minister in the 1990s and once was seen as a possible successor to Boris Yeltsin, Russia's first elected president. After Putin was first elected in 2000, Nemtsov became one of the most vocal critics of his rule. He helped organize street protests and exposed official corruption.
He was one of the organizers of the Spring March opposition protest set for Sunday, which comes amid a severe economic downturn in Russia caused by low oil prices and Western sanctions.
Nemtsov said during a radio interview just before his death that it was hard to live under constant intimidation and pressure.
"I won't hide the fact that the opposition is under strong pressure," he said. "Lies are spread about the people, and one has to be a very strong person to cope with all this. I know this from my own experience."
Peter Leonard in Kiev, Ukraine contributed to this report.
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