MOSCOW (AFP) - Thousands of mourners on Tuesday filed past the coffin of slain Russian opposition leader Boris Nemtsov, bidding farewell to a charismatic activist whose brazen assassination shocked the country.

The 55-year-old former deputy prime minister, longtime Putin critic and anti-corruption crusader who was gunned down near the Kremlin late Friday, will be buried in Moscow later on Tuesday.

The European Union condemned what it called "arbitrary" bans imposed on several prominent European figures whom Russia had blocked from attending the funeral.

Nemtsov's muder marked the most high-profile assassination of President Vladimir Putin's rule, sent shock waves across Moscow and triggered international condemnation led by US President Barack Obama.

Thousands of mourners, some clutching flowers and candles, formed a huge queue outside the Andrei Sakharov rights centre in central Moscow where Nemtsov's body lay in state.

As Bach's St Matthew Passion played in the background, well-wishers filed past the coffin covered with flowers, many crossing themselves and some weeping.

Nemtsov's mother Dina Eidman, who turned 87 on Tuesday, his children, widow and former partners and friends stood by the casket.

Along with ordinary Russians were dignitaries including former president Boris Yeltsin's widow Naina, respected former finance minister Alexei Kudrin, billionaire Mikhail Prokhorov, Anatoly Chubais, the architect of post-Soviet Russia's mass privatisations, former prime minister turned opposition leader Mikhail Kasyanov and Deputy Prime Minister Arkady Dvorkovich.

Former British prime minister John Major and US ambassador John Tefft were among the foreign dignitaries to arrive.

Putin will not attend the funeral, his spokesman Dmitry Peskov told AFP.

'PUTIN IS TO BLAME'

Many mourners blamed the assassination on Putin, saying the Kremlin - locked in a bitter confrontation with the West over Ukraine - whipped up hatred against dissenters.

"We came because we feel ashamed of our country, of our people, that we let such a thing happen," said one of the mourners, Dmitry Afanasyev. "Putin is to blame. But we are too." "It's a shock. It's the system that killed him," said another mourner, Vladimir Shlamin.

A journalist and ally of Nemtsov, Vladimir Kara-Murza, wept as he said friends had sought to convince the politician to leave the country but he had refused.

"He could have lived a comfortable and easy life. He chose a different path," said opposition activist Ilya Yashin. "He has left us as a hero." The top leader of the beleaguered opposition, Alexei Navalny, who is serving a short stint in jail, had been denied permission to attend the funeral.

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

'RUSSIAN PATRIOT, FRIEND OF UKRAINE'

Putin, whose rule has seen the steady suppression of independent media, opposition political parties and independent-minded business figures, called the murder a "contract killing" and said it was a provocation.

He promised an all-out effort to catch the perpetrators.

Friends said Nemtsov 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.

A posthumous award, the Order of Freedom, was bestowed on Nemtsov by Ukrainian President Petro Poroshenko.

"For us Ukrainians, Boris will forever remain a Russian patriot and a friend of Ukraine," he said. "He proved with his life that one can be both." Investigators said they had examined security camera footage and questioned witnesses and were going to carry out a number of forensic tests.

Authorities were also searching the Moskva River under the bridge where Nemtsov was killed, an AFP journalist saw.

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 RETURNS TO KIEV'

Nemtsov's 23-girlfriend Duritska - the chief witness in the murder - was allowed to return to Kiev.

Investigators on Tuesday said that Duritska was a key witness and "could know some circumstances that have significance." After the ouster of Ukraine's Kremlin-backed president Viktor Yanukovych ties between Moscow and Kiev were plunged into crisis, as Russia seized Crimea and a separatist insurgency erupted in Ukraine's east.

Since Putin came to power in 2000, a string of prominent opposition figures have been murdered including journalist Anna Politkovskaya and activist Natalya Estemirova.

The Russian opposition and Western leaders have called for a full and transparent probe into the murder of Nemtsov, who served as 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 opposition rallies since anti-Putin demonstrations in 2011 and 2012.

Investigators offered several possible motives, including that Nemtsov might have become a "sacrificial victim" of the opposition in order to sow instability.

The murder took place in Russia's most heavily policed area, right next to the Kremlin walls.

Some observers have suggested that low-level criminals, not professional hit men, may have carried out the killing.