DUSHANBE, February 28, 2015, Asia-Plus -- Boris Nemtsov, an outspoken critic of Russian President Vladimir Putin and Russia''s role in the Ukraine crisis, was shot dead on Bolshoi Moskvoretsky Bridge close to the Kremlin late on the evening of February 27.

Russian media sources report Nemtsov was shot four times in the back by assailants in a light-colored car as he walked across a bridge over the Moskva River in central Moscow with a female companion, who was unharmed, at the time of the killing.

The woman was reportedly a Ukrainian national and was taken for questioning by police. A representative of Russia’s investigative committee at the scene of the murder said investigators were considering “all possible versions.”

Reuters says there was no immediate claim of responsibility and former Soviet President Mikhail Gorbachev cautioned against jumping to conclusions.  “Certain forces will try to use the killing to their own advantage. They are thinking how to get rid of Putin,” he said.

Putin condemned the “brutal” slaying and took the investigation under presidential control, saying it could have been a contract killing and a “provocation” on the eve of a big opposition protest Nemtsov had been due to lead on Sunday.

U.S. President Barack Obama reportedly called for a prompt, impartial and transparent investigation to ensure those responsible were brought to justice for the “vicious killing.”

The Guardian reports Britain has said it will follow closely investigations into the killing.  A Foreign Office spokeswoman said: “We are shocked and saddened by news … We deplore this criminal act.  Those responsible must be brought to justice.  We will continue to follow the situation closely.”

Just hours before his death, Nemtsov had appeared on Ekho Moskvy radio calling on Muscovites to attend an opposition march planned for Sunday. The march against Putin’s government and the war in Ukraine was due to take place in a suburb of Moscow.  One of the other organizers of the march, Alexei Navalny, was jailed on February 19 for 15 days.

Organizers of Sunday''s march were divided after Nemtsov''s death over whether to hold the rally or turn it into a demonstration of mourning in central Moscow.

Opposition politicians and some foreign officials said the murder showed the problems faced by dissident voices in a country where Putin demands total loyalty, Reuters says.

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.