Russian opposition leader Boris Nemtsov was shot dead just meters away from Kremlin, central Moscow on Friday.

Nemtsov, 55, was walking across a bridge over the Moskva River with a Ukrainian woman, when gunmen on board a white car shot him four times at the back. Nemtsov's companion was unhurt.

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Police cars immediately cordoned off the bridge just steps away from the Kremlin and the Red Square, as an investigation into the killing swiftly followed.

Nemtsov is a known critic of President Vladimir Putin as well as of Russia's involvement in the Ukraine war. So far, he stands as the most prominent opposition figure killed under Putin's 15-year regime.

"That a leader of the opposition could be shot beside the walls of the Kremlin is beyond imagination. There can be only one version: that he was shot for telling the truth," said Mikhail Kasyanov, an opposition leader and a former prime minister who served under Putin.

Based on the initial probe, Nemtsov, while walking out from a restaurant in Red Square was quoted as saying, the president might want him dead over his opposition to Russia's war with Ukraine.

Kremlin was quick to deny any involvement in the killing.  Putin immediately issued a statement condemning the brutal slay, but ordered the investigation to be placed under his control. 

The president said, Nemtsov's death could be part of a grand plan to spark outrage on the eve of a big opposition protest on Sunday which Nemtsov is supposed to lead.

But for many, Nemtsov's death was linked to the tough treatment the opposition has been getting under Putin's third term.  Several top Kremlin critics have been put behind bars or have fled the country after staging mass protests.

Fellow opposition politician Ilya Yashin said that Nemtsov had been receiving threats before his death.

"Boris periodically received anonymous threats on social networking sites... Boris was worried." Yashin said.  Despite the threats, Yashin added, Nemtsov never wanted additional security saying if there is a plot to kill him, then he will be killed.

U.S. President Barack Obama has called for an immediate and transparent probe into the killing saying the perpetrators must be 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," Obama said. 

Obama and Putin had been at odds over many issues particularly the war in Ukraine.

Obama called Nemtsov a "dedicated and eloquent" defender of Russian people's rights.  Obama praised Nemtsov for his tireless struggle to fight corruption in Russia.

Flowers were offered on the sidewalk of the Moskvoretsky Bridge as supporters pay their respects to the slain opposition leader. 

TagsBoris Nemtsov, Russia, Moscow, Ukraine