The Ukrainian model girlfriend of slain Russian opposition leader Boris Nemtsov complained Monday she was being blocked from leaving Moscow, as authorities vowed to solve the most shocking political assassination of Vladimir Putin's rule.

The funeral for Nemtsov, a 55-year-old former deputy prime minister, longtime Putin critic and anti-corruption crusader, was to be held Tuesday in the Russian capital's Troekurovskoye cemetery.

His body was to lie in state, starting at 0700 GMT, at the Andrei Sakharov rights centre, the RIA-Novosti news agency quoted Nemtsov colleague Konstantin Merzlikin as saying.

Nemtsov was shot dead shortly before midnight Friday while walking across a bridge just a short distance from the Kremlin together with his girlfriend, 23-year-old Ganna Duritska.

Putin, whose rule has seen the steady suppression of independent media, non-Kremlin controlled political parties and opposition-minded business figures, called the murder a "contract killing" and said it was a provocation. Soon after the killing, Putin promised an all-out effort to catch the perpetrators.

On Monday, Foreign Minister Sergei Lavrov pledged that the "heinous crime" would be "fully investigated."

[PHOTO GALLERY]: TENS OF THOUSANDS MARCH FOR BORIS NEMTSOV DEATH

A reward of three million rubles ($48,000) was offered for information on Nemtsov's death, a substantial amount in Moscow, where the average monthly salary is 60,000 rubles ($960).

Girlfriend witnessed slaying

Duritska, the chief witness, said she had given all the information she could to investigators but they were preventing her from leaving Russia, citing concerns for her security.

"For three days they have escorted me in police cars to the Investigative Committee," Duritska told the independent Dozhd television channel, referring to the agency in charge of the probe.

"They don't explain when I will be let go or for what reason I am kept here."

Ganna's mother Inna Duritska, who lives in Kiev, told AFP that her daughter's de facto house arrest meant investigators might be preparing to make her a pawn in the deepening Russia-Ukraine crisis.

"I am afraid that she will be accused of this murder because they need the Ukrainian trace. They can invent anything they want," she said.

Moscow and Kiev have been foes since the ouster last year of Kremlin-backed president Viktor Yanukovych and Russia's seizure of the Ukrainian Crimea province.

Ukraine's current pro-Western government has also been battling a pro-Russian insurgency in the east since April.

Nemtsov was said by friends to have been working on a report containing what he said was proof of secret Russian military involvement in the bloody uprising by pro-Moscow militias in eastern Ukraine.

He had also spoken of his fear of being killed in Russia, where a string of other prominent opposition figures have been murdered since Putin came to power 15 years ago.

But there was no imminent sign of danger on Friday when he was fatally shot in the back, said Duritska, who was uninjured in the apparently well-prepared shooting.

Speaking via a fuzzy Skype connection from a Moscow apartment, Duritska said she did not see where the assassin came from. But she did notice a light-coloured car quickly drive off, she said.

She said she was immediately taken in for questioning which lasted through the night.

'A sacrifice'

Shocked opposition figures in Russia and Western leaders called for a full and transparent probe into the murder of Nemtsov, who served as Boris Yeltsin's first deputy prime minister in the 1990s.

On Sunday, tens of thousands of people joined a memorial march in Moscow, numbers not seen at an opposition-linked event since mass anti-Putin rallies in 2011 and 2012.

The powerful Investigative Committee leading the probe has offered several possible motives, including that the country's opposition could itself have ordered the hit on Nemtsov as "a sacrifice".

The murder took place in one of the most heavily policed areas of Moscow. However, some Russian media reports suggested that low-level criminals, not professional hit-men, may have carried out the killing.

"Participants in the investigation are only sure of one thing -- that the killers were not professionals," said Kommersant.

The broadsheet said they used ammunition that was years old and possibly an unreliable home-made weapon.

The murderer -- or murderers -- fired four bullets into Nemtsov's back and several more were found at the scene.

The weapon may have been a Makarov pistol, used by Russian armed forces, or an easily-acquired Izh gas pistol, converted to fire live rounds, the newspaper said.

A converted weapon of this type was used in 2006 to kill journalist and Kremlin critic Anna Politkovskaya at her home in central Moscow. That murder remains largely unsolved.

Fellow opposition leader Alexei Navalny said Nemtsov would almost certainly have been under some form of surveillance at the time of his death, given the two had been planning a major rally in Moscow on Sunday.

Nemtsov was shot a few hours after urging listeners on Echo of Moscow radio to attend the Sunday opposition rally called to demand an end to the war in Ukraine.