The death of Kremlin critic Boris Nemtsov, who was killed on a Moscow street on Friday night, is causing speculation over who is behind the seemingly targeted killing. Russian opposition leaders and President Vladmir Putin's government and its supporters are exchanging blame for the opposition leader's sudden death.

The authorities have suggested the opposition itself may have been behind his shooting in an attempt to create a martyr and unite the fractured movement.

His supporters have blamed the authorities.

"If we can stop the campaign of hate that's being directed at the opposition, then we have a chance to change Russia. If not, then we face the prospect of mass civil conflict," Gennady Gudkov, an opposition leader, told Reuters.

"The authorities are corrupt and don't allow any threats to them to emerge. Boris was uncomfortable for them."

'If we can stop the campaign of hate that's being directed at the opposition, then we have a chance to change Russia.' - Gennady Gudkov, an opposition leader

The Associated Press spoke to Ilya Yashin, a friend and fellow opposition leader, who 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 Nemtsov and did not agree with the government," Yashin told The Associated Press. "I hope we won't get scared, that we will continue what Boris was doing."

His murder has divided opinion in a country where for years after the Soviet Union collapsed many yearned for the stability later brought by former KGB agent Putin.

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Russian opposition politician Boris Nemtsov was gunned down late Friday. (Dmitry Korotayev/Epislon/Getty Images)

A small but active opposition now says Putin's rule has become an autocracy that flouts international norms after Russia seized Ukraine's Crimea peninsula last year, fanned nationalism over the separatist war in eastern Ukraine and clamped down on dissent.

"[Nemtsov] was harmful to the authorities, but the authorities themselves are criminal. The authorities have trampled on all international rights, seized Crimea, started war with Ukraine," said Yuri Voinov, a physicist.

$61K reward offered by Russian investigators

Putin has vowed to pursue those who killed Nemtsov, calling the murder a "provocation."

National investigators who answer to the Russian leader offered a three-million-ruble reward, nearly $61,000, for information on Nemtsov's death. They say they are pursuing several lines of inquiry, including the possibility that Nemtsov, a Jew, was killed by radical Islamists or that the opposition killed him to blacken Putin's name.

Nemtsov's funeral is due to be held on Tuesday in Moscow.

Boris Nemtsov

Opposition rallies are rarely granted permits in Moscow. Critics of Putin maintain its part of a larger effort to silence anti-Putin voices in the Russian capital. (Maxim Shemetov/Reuters)

Putin's opponents say such suggestions, repeated over pro-Kremlin media, show the cynicism of Russia's leaders as they whip up nationalism, hatred and anti-Western hysteria to rally support for his policies on Ukraine and deflect blame for an economic crisis.

"We are told on TV that a conspiracy by the West and those among us who have sold out to them are behind our poverty. People should throw away the TV set and go to protest," said Olga, 42, who declined to give her last name.

Some Muscovites have accepted the official line and appear to agree that the opposition, struggling to make an impact after a clampdown on dissent in Putin's third spell as president, might have killed one of their own.

"The authorities definitely do not benefit from this. Everybody had long forgotten about this man, Nemtsov ... It is definitely a 'provocation,'" said one Moscow resident, who gave his name only as Denis.

Some young people walking in central Moscow asked: "Who is Nemtsov anyway?"

Planned to publish report criticizing Putin

Nemtsov, who was 55, was one of the leading lights of a divided opposition struggling to revive its fortunes, three years after mass rallies against Putin failed to prevent him returning to the presidency after four years as prime minister.

Boris Nemtsov march Moscow

The march is show of defiance for a fledgling opposition, whose leaders have been purged from Russian state media. (Sergei Karpukhin/Reuters)

With an athletic build and characteristic mop of curly hair, Nemtsov had been a face of the opposition for years, along with anti-corruption blogger Alexei Navalny and former prime minister Mikhail Kasyanov, though no one figure has succeeded in uniting the ranks of opposition-minded Muscovites.

The opposition has little support outside big cities and Putin has now been Russia's dominant leader since 2000, when ailing president Boris Yeltsin chose him as his successor, a role Nemtsov had once been destined to play.

Even many of Putin's opponents have little doubt that he will win another six years in power at the next election, due in 2018, despite a financial crisis aggravated by Western economic sanctions over the Ukraine crisis and a fall in oil prices.

Many opposition leaders have been jailed on what they say are trumped-up charges, or have fled the country.

Boris Nemtsov dead

A woman visits the site where Boris Nemtsov was recently murdered, in central Moscow. Russia's Investigative Committee is pursuing several lines on inquiry following the killing. (Sergei Karpukhin/Reuters)

Nemtsov, a fighter against corruption who said he feared Putin may want him dead, had hoped to start the opposition's revival with a march he had been planning for Sunday against Putin's economic policies and Russia's role in east Ukraine.

The Kremlin denies sending arms or troops to Ukraine.

In a change of plan, the opposition said Moscow city authorities had allowed a march of up to 50,000 people alongside the River Moskva to commemorate Nemtsov's death.

Ukrainian President Petro Poroshenko said Nemtsov had told him about two weeks ago that he planned to publish evidence of Russian involvement in Ukraine's separatist conflict.

30,000 mourners attend march

Tens of thousands of Russians marched through central Moscow on Sunday, carrying banners declaring "I am not afraid" and chanting "Russia without Putin" in memory of Nemtsov.

Families, the old and young walked slowly, with many holding portraits of the opposition politician and former deputy prime minister who was shot dead while walking home from a nearby restaurant on Friday night.

Police said 30,000 people attended the march. The organizers put the numbers at tens of thousands, but attendance appeared smaller than the 50,000 people the opposition had hoped for.

Reuters reporters at the march estimated the numbers in the tens of thousands.

Foreign Affairs Minister Rob Nicholson told the Canadian Press that Canada stands with those Russians protesting the killing. Nicholson said he doesn't know who is to blame for the shooting.

"I understand and support those who are taking to the street," he told The Canadian Press in his first interview since replacing John Baird last month. "They saw progress after the end of communism, and now I'm sure that many of them are very worried about what Putin is doing."

Solidarity rallies were held in Vancouver and Toronto.

In Moscow, people walked in the rain within view of the Kremlin's red walls and past the spot, now covered in flowers, where Nemtsov was shot dead.

Some carried large banners carrying Nemtsov's face reading "Heroes Never Die", the same slogan used in Ukraine to celebrate more than 100 people killed in protests that overthrew Moscow-leaning Ukraine president Viktor Yanukovich a year ago.

One elderly woman, her hair tucked into a woollen cap, held up a hand-written sign to cover her face: "It's a geopolitical catastrophe when a KGB officer declares himself president for life. Putin resign!"

Ukrainian parliament member detained

Moscow police have detained a member of Ukraine's parliament as he was taking part in the march, the Associated Press reported. Ukraine's parliament has protested the detention.

The federal Investigative Committee said Alexei Goncharenko was being questioned Sunday about his alleged involvement in a fire that broke out last year in his home city Odessa between pro-Ukrainian and pro-Russian demonstrators. Dozens died in the fire, including some Russian citizens.

The speaker of Ukraine's parliament, Volodymyr Groisman, said the detention was a violation of international law because Goncharenko has diplomatic immunity. The speaker urged Ukrainian Foreign Minister Pavlo Klimkin to take urgent measures to have the parliament member released.

For many Russians, the Odessa fire remains one of the more painful episodes of the Ukraine conflict.

With files from the Associated Press, the Canadian Press