28 February 2015
Last updated at 21:39
Thousands of people have paid tribute with candles and flowers at the site in Moscow where leading Russian opposition politician Boris Nemtsov was shot dead.
An opposition march which Mr Nemtsov planned in Moscow for Sunday has now been turned into a memorial rally.
President Vladimir Putin condemned Friday’s killing as “vile and cynical” and vowed to find the killers.
But Mr Nemtsov’s allies said this was a political killing linked to opposition to Mr Putin and the Ukraine conflict.
‘They won’t stop us’
As night fell, flowers were piled up a metre high and two metres wide on the Great Moskvoretsky Bridge.
Placards read: “We are all Nemtsov” and “Je Suis Boris” – the latter a reference to the Je Suis Charlie messages of support following the Charlie Hebdo killings in Paris last month.
At the scene: Sarah Rainsford in Moscow
There is now a mountain of flowers on the spot where Boris Nemtsov was shot, right next to the Kremlin.
People have tied bouquets to the wall of the bridge, and to lampposts. They have brought photographs, handwritten posters and poems. One proclaimed “Je Suis Boris”, another read in red ink, “Boris, they were frightened of you.” To one side, a man stood alone with a banner: “4 bullets in you are 4 bullets in me”.
Many here believe that Boris Nemtsov was killed for his political views. They blame President Putin for fanning the flames of nationalism to dangerous heights, where political dissidents are now seen as traitors – the “enemy within.”
Amidst the flowers and candles are flyers for the opposition rally Boris Nemtsov had been helping to organise on Sunday. Instead, the crowds will gather for a march of mourning.
One of those attending, Alexander Badiyev, said: “This was ordered by Putin, without a shadow of a doubt. They have shown us what the fate will be of all those who are against them.”
Opposition activist Mark Galperin said: “People are afraid to support our movement. Opposition activists receive threats every day and Boris was no exception. But they won’t stop us.”
Former Yabloko party leader Grigory Yavlinsky said: “The political responsibility for this murder lies with the authorities and personally President Putin.”
A number of European ambassadors laid wreaths.
Former UK ambassador to Russia, Sir Tony Brenton, told the BBC there was now “an atmosphere of political hysteria in Russia”.
“And that is an atmosphere which has generated a lot of really rather nasty right-wing ex-soldiers’ groups to come to the surface. And it is quite possible that it is a group like that that was responsible for Boris’s death.”
Russia’s Investigative Committee said it was looking into a number of possible motives, including Mr Nemtsov’s opposition to the Ukraine war, his political and personal life, Islamic extremism or an attempt to destabilise the state.
A number of pro-government figures suggested Mr Nemtsov had been made a sacrificial victim to show the state in a bad light.
Ramzan Kadyrov, the Putin-backed leader of Chechnya, blamed “Western special services, trying by any means to create internal conflict in Russia”.
Presidential spokesman Dmitry Peskov said Mr Putin had noted “that this cruel murder has all the makings of a contract hit and is extremely provocative”.
Others suggested there could have been personal enmity over his private or business life.
Lawmaker Vladimir Zhirinovsky said the murder could have been connected to Nizhny Novgorod where Mr Nemtsov had been regional governor in the 1990s.
Mr Nemtsov was reportedly preparing documents on Russian military involvement in Ukraine.
Ukrainian President Petro Poroshenko said: “Boris had declared he would provide clear evidence of Russian armed forces’ participation in [the war] in Ukraine. Somebody was afraid of this… They killed him.”
US President Barack Obama condemned the killing as a “brutal murder”.
The Russian government must conduct a “prompt, impartial and transparent investigation”, the US president urged.
‘He’ll kill you’
Mr Nemtsov, 55, had been dining at a restaurant with his girlfriend Anna Duritskaya on Friday night.
They left together to walk to his flat, crossing the bridge, where a white car drew up and Mr Nemtsov was shot four times with a pistol at around 23:40 (20:40 GMT).
Mr Nemtsov served as first deputy PM under Boris Yeltsin but fell out of favour with Mr Putin
A group of EU ambassadors to Russia lays flowers in Moscow
Footage on Russian TV showed a white Lada Priora car in the area but there was no confirmation it was the one involved. One shot showed someone running along the road and jumping into the waiting car, which sped off.
In a telegram to Mr Nemtsov’s mother, published on the Kremlin website, Mr Putin vowed to bring the killers to justice. He praised Mr Nemtsov’s openness and honesty.
Mr Nemtsov served as first deputy PM under President Boris Yeltsin in the 1990s but fell out of favour with Mr Putin and became an outspoken opponent.
He told the weekly Sobesednik recently that his mother was worried about him.
“She is more worried about Putin than Ukraine. Every time I call her, she gives me a talking-to: ‘When will you stop being rude about Putin? He’ll kill you.'”
Moscow’s authorities have agreed a march of up to 50,000 people can be held on Sunday.
It will begin on Kitaigorodsky Proezd at 15:00 local time (12:00 GMT) and pass the site of the killing.
Violent deaths of Putin opponents
April 2003 – Liberal politician Sergey Yushenkov assassinated near his Moscow home
July 2003 – Investigative journalist Yuri Shchekochikhin died after 16-day mysterious illness
July 2004 – Forbes magazine Russian editor Paul Klebnikov shot from moving car on Moscow street, died later in hospital
October 2006 – Investigative journalist Anna Politkovskaya shot dead outside her Moscow apartment
November 2006 – Former Russian spy Alexander Litvinenko died nearly three weeks after drinking tea laced with polonium in London hotel
March 2013 –Boris Berezovsky, former Kremlin power broker turned Putin critic, found dead in his UK home