Carrying flowers, portraits and signs that said "I am not afraid," more than 20,000 people marched in Moscow to mourn opposition politician Boris Nemtsov, whose killing on the streets of the capital has shaken Russia's opposition.

Russian President Vladimir Putin has marginalised and intimidated his political opponents, jailing some and driving others into exile, since mass anti-Putin protests swept Moscow in 2011 and 2012. Mr Nemtsov, 55, was among the few prominent opposition figures who had refused to be cowed.

The mourners marched to the bridge near the Kremlin where Mr Nemtsov was gunned down shortly before midnight on Friday. Chanting "we'll not forget, we'll not forgive," the crowd filled the road along the Moscow River embankment. Many waved Russian flags.

The mood was sombre, with heavy security. Police helicopters flew overhead and police boats patrolled the river.

Ilya Yashin, a friend and fellow opposition leader, said he hoped the killing would not frighten people.

"Essentially it is an act of terror. It is a political murder aimed at frightening the population, or the part of the population that supported Mr Nemtsov and did not agree with the government," Yashin said. "I hope we won't get scared, that we will continue what Boris was doing."

Mikhail Kasyanov, a former prime minister who joined the opposition, told the crowd the killing should be a turning point for Russia "for the simple reason that people who before thought that they could quietly sit in their kitchens and simply discuss problems within the family, now will start reconsidering everything that's going on in our country".

Russia's federal investigative agency said it was looking into several possible motives for Mr Nemtsov's killing.

The first possibility, the Investigative Committee said, was that the killing was aimed at destabilising the political situation in Russia and Nemtsov was a "sacrificial victim for those who do not shun any method for achieving their political goals".

This suggestion echoed comments by Mr Putin's spokesman and other Russian politicians that the attack was a "provocation" against the state. The consensus of political commentators on state television was that the killing served the interests of Russia's enemies.

Mr Yashin, however, said Russia's leadership and specifically Mr Putin bore full political responsibility for Mr Nemtsov's death.

"It was President Putin who created this atmosphere of hate in our country, the atmosphere of intolerance, which one way or another materialised in the bullet that killed my friend Boris Nemtsov," Mr Yashin said.

Kremlin propaganda had identified Mr Nemtsov as among the leaders of a "fifth column," painting him and other opposition figures as traitors in the service of a hostile West.

As a former deputy prime minister and long-time politician, Mr Nemtsov retained strong ties among Russia's political and business elite, making his killing additionally shocking.

He was killed just hours after a radio interview in which he denounced Mr Putin's "mad, aggressive and deadly policy of war against Ukraine." Mr Nemtsov was working on a report presenting evidence that he believed proved that Russian servicemen were fighting with the separatists in Ukraine, despite official denials.

Mr Yashin said it was unlikely the report would ever be published because investigators who searched Mr Nemtsov's apartment after his death took away his laptop.

TV Centre, a station controlled by the Moscow city government, broadcast a poor-resolution video from one of its web cameras that it said shows Mr Nemtsov and his female companion shortly before the killing. The station, which superimposed its own time code on the footage, circled figures that it said were Mr Nemtsov and the woman walking across the bridge on a rainy night. A snowplough that moved slowly behind the couple obscured the view of the shooting. TV Centre then circled what it said was the suspected killer jumping into a passing car. The authenticity of the video could not be independently confirmed.

Investigators said they were again questioning the woman, Ukrainian citizen Anna Duritskaya, who was with Mr Nemtsov when he was killed. LifeNews, a television station with ties to the security services, said Ms Duritskaya told investigators that she was in shock and could not remember what the killer looked like or the car he was in.

The Interior Ministry said it was offering 3 million rubles for information about the killing.

US Secretary of State John Kerry said the United States had no intelligence on who was being the shooting. "The bottom line is we hope there will be a thorough, transparent, real investigation, not just of who actually fired the shots, but who, if anyone, may have ordered or instructed this or been behind this," Mr Kerry said on ABC's "This Week."

Another mourning march for Mr Nemtsov was held earlier in St Petersburg, drawing what police estimated was 6,000 people.

Nelly Prusskaya, a 66-year-old doctor, said she came to pay her respects to Mr Nemtsov.

"I also came to say that I'm against the war in Ukraine," she said. "I'm against political murders."

Most Russians support Mr Putin's policy in Ukraine. Since the annexation of Crimea a year ago, support for Mr Putin has been more than 80 per cent.

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