MOSCOW (AFP) - Mourners for slain Russian opposition leader Boris Nemtsov prepared to gather for his funeral in Moscow on Tuesday, as Polish and Latvian officials said they had been barred from entering Russia on the eve of the ceremony in retaliation for EU sanctions over Ukraine.

The funeral for Mr Nemtsov, a 55-year-old former deputy prime minister, long-time Vladimir Putin critic and anti-corruption crusader who was shot dead in central Moscow four days ago, will be held in the Russian capital's Troekurovskoye cemetery.

His body was to lie in state starting at 7.00AM GMT at the Andrei Sakharov rights centre, the RIA-Novosti news agency quoted Nemtsov colleague Konstantin Merzlikin as saying.

Senior officials from Poland and Latvia who sought to attend the funeral said on Monday that they had been denied entry into Russia.

Polish Senate speaker Bogdan Borusewicz was refused entry by Moscow in reprisal for EU sanctions against Russia's upper house of parliament speaker Valentina Matviyenko, the Polish foreign ministry said.

Latvian MEP Sandra Kalniete told AFP she had also been refused entry into Russia at Moscow's Sheremetyevo airport, but was not given a reasonable explanation for the ban.

"Since I have always taken a clear and explicit language on Russia's role in Ukraine, I had suspicions that it could happen," she said.

Polish Deputy Foreign Minister Konrad Pawlik will attend the funeral, according to Polish media, while Lithuanian Foreign Minister Linas Linkevicius will also be present, his spokesman said.

Mr Nemtsov was gunned down shortly before midnight last Friday while walking across a bridge just a short distance from the Kremlin with his Ukrainian model girlfriend Ganna Duritska.

Mr 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, Mr Putin promised an all-out effort to catch the perpetrators.

Girlfriend returns to Kiev

Late Monday, 23-year-old Duritska - the chief witness in the murder - arrived in Kiev, her lawyer confirmed, as authorities vowed to solve the most shocking political assassination to take place during Putin's rule.

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

A reward of three million rubles (S$65,535) was offered for information on Nemtsov's death, a substantial amount in Moscow, where the average monthly salary is 60,000 rubles.

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

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

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.

Friends of Mr Nemtsov said he had been working on a report containing what he described as 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 Mr 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, Ms Duritska - who was uninjured in the apparently well-planned shooting - said earlier.

Speaking via a fuzzy Skype connection from a Moscow apartment, Ms 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 have called for a full and transparent probe into the murder of Mr 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 Investigative Committee leading the probe has offered several possible motives, including that the country's opposition could itself have ordered the hit on Mr 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 Mr Nemtsov's back and several more were found at the scene.