Russia bids farewell to slain Putin critic Boris Nemtsov

Thousands of mourners have 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, long-time critic of President Vladimir Putin and anti-corruption crusader who was gunned down outside of the Kremlin walls late on Friday, will be buried in Moscow later on Tuesday.

The most shocking political murder of Putin's rule sent shockwaves across Russia and triggered international condemnation led by US President Barack Obama.

Former British prime minister John Major was among the first to arrive at the Andrei Sakharov rights centre in central Moscow early on Tuesday, where Nemtsov's body lay in state.

The slain politician's mother, Dina Eidman, who turned 87 on Tuesday, arrived wearing a black headscarf.

Russian Deputy Prime Minister Arkady Dvorkovich carried a bouquet of red roses as he walked in, as did former prime minister Mikhail Kasyanov, himself now an opposition leader.

Putin was not due to attend the funeral, with the Kremlin sending a low-level official instead.

"We came because we feel ashamed of our country, of our people, that we let such a thing happen. It's our cruelty, our fear," 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. "We are against the system that kills great people."

Bach played in the background as well-wishers came up to the coffin to lay flowers, many crossing themselves.

Polish Deputy Foreign Minister Konrad Pawlik was also set to attend as well as Lithuanian Foreign Minister Linas Linkevicius.

But several senior officials from Poland and Latvia said Monday they had been denied entry into Russia to attend the funeral in retaliation for EU sanctions over Ukraine.

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

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.

Investigators said Tuesday they had examined security camera footage and questioned witnesses and were going to carry out a number of forensic tests.

A reward of three million roubles ($A61,373.23) was offered for information on Nemtsov's death, a substantial amount in Moscow, where the average monthly salary is 60,000 roubles ($A1,227.46).

Late Monday 23-year-old Duritska - the chief witness in the murder - was allowed to return to Kiev.

She said she had given all the information she could to investigators and did not plan to attend the funeral.

Duritska said earlier 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.

Friends of 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.