Major Russian opposition leader and former deputy prime minister Boris Nemtsov was shot dead Friday night in central Moscow.
The Interior Ministry said Nemtsov was shot four times from a passing car as he walked across a bridge right next to the Kremlin.
The official Itar-Tass news agency reported President Vladimir Putin was "immediately" informed about Nemtsov's murder and said Kremlin will oversee the investigation.
A Putin spokesman said the president said it looks like a contract killing that could be a provocation ahead of an opposition march set for Sunday.
President Barack Obama said in a statement, "The United States condemns the brutal murder of Boris Nemtsov, and we call upon the Russian government to conduct a prompt, impartial, and transparent investigation into the circumstances of his murder and ensure that those responsible for this vicious killing are brought to justice.
"Nemtsov was a tireless advocate for his country, seeking for his fellow Russian citizens the rights to which all people are entitled. I admired Nemtsov’s courageous dedication to the struggle against corruption in Russia and appreciated his willingness to share his candid views with me when we met in Moscow in 2009.
We offer our sincere condolences to Boris Efimovich’s family, and to the Russian people, who have lost one of the most dedicated and eloquent defenders of their rights."
Watch video of Nemtsov rallying for Ukraine peace
Accusing the state
Speaking with VOA by phone from his home in Cambridge, England, Vladimir Bukovsky, a famous Soviet dissident and personal friend of Boris Nemtsov, said, "I suspect it’s a state murder."
Former Russian Prime Minister Mikhail Kasyanov gave an exclusive statement to VOA Russian reporter Danila Galperovich.
"It will never be forgiven ... It is a premeditated, carefully planned assassination. It is shocking. That is how it has ended up in our country," said Kasyanov. "An opposition leader has been killed in the heart of Moscow, near the Kremlin wall. It is unbelievable. We will do everything we can to hold those bastards accountable. We will continue our work and our fight to make Russia free and democratic country".
VOA Moscow correspondent Daniel Schearf said the march will likely become much bigger than planned and says the opposition will probably accuse the Kremlin of being involved in Nemtsov's murder.
Nemtsov was a deputy prime minister in the 1990s and many Russian observers predicted he would succeed then President Boris Yeltsin.
After President Yeltsin chose Vladimir Putin as his successor and Putin's subsequent election in 2000, Nemtsov became one of Russia's sharpest and most outspoken Putin critics, especially since last year's uprising in Ukraine.
In September, Nemtsov told VOA's Schearf that Putin wants revenge for Ukraine's overthrow of its pro-Russian president.
He said Purtin fears that what happened in Ukraine could happen in Russia and sees a pro-European Ukraine as a threat to his own power.
“He lies in revenge for Ukraine's revolution, when Ukrainians took to the streets and dethroned the corrupt thief President Yanukovich.[President Putin] is afraid it could be repeated in Russia. And, besides, he thinks if Ukraine is successful on the European path it is a threat to his own power," said Nemtsov.
In an op-ed titled “Why does Putin wage war with Ukraine?” published in the Kyiv Post in September, Nemtsov blasted the Russian president, writing “Moreover, Ukraine chose the European way, which implies the rule of law, democracy and change of power. Ukraine's success on this way is a direct threat to Putin's power because he chose the opposite course – a lifetime in power, filled with arbitrariness and corruption.”