A rally to remember Russian opposition leader Boris Nemtsov — gunned down five years ago this week — is taking place in Moscow.

You can watch footage of it in the video player, above.

Earlier this week, western allies marked the anniversary of Nemtsov's murder by calling on authorities to find the mastermind behind the crime.

Nemtsov, a former Russian MP, governor and deputy prime minister, was gunned down near the Kremlin on February 27, 2015.

The 55-year-old had become a leading opposition figure and a vocal critic of President Vladimir Putin. At the time of his death, he was working on an investigative report on Russian military involvement in Ukraine.

'Impunity for attacks'

Mourners who gathered on the Bolshoy Moskvoretsky Bridge on Thursday to pay tribute to the slain opposition figure included French, British and US diplomats.

"On the fifth anniversary of Boris Nemtsov's murder, we again call on Russian authorities to investigate those who organised and ordered this horrific crime," Rebecca Loss, spokesperson at the US embassy in Moscow wrote on Twitter.

Britain said in a statement: "We urge Russia to renew efforts to identify the person or persons who ordered the murder of Boris Nemtsov five years ago".

"Holding the instigators of this horrific crime to account would go some way to ending the culture of impunity for attacks on political activists, journalists and human rights defenders in the Russian Federation," it added.

'Torture and unfair trial'

Five men have been arrested and sentenced to between 11 and 19 years of jail for carrying out the assassination but the instigator(s) remains at large and the investigation has stalled.

A report from the Organisation for Security and Co-Operation in Europe (OSCE) released on Thursday deplored that, five years after the slaying, "no motive has been confirmed."

"The question of who commissioned the assassination, and why, thus remains open," the report noted.

Margareta Cerdefelt, the Swedish MP and OSCE Special Rapporteur who authored the report told Euronews that "the investigation done by Russia is incomplete, there are numerous inconsistencies".

"In the judicial process, the defendants were subject to torture and (were) denied a fair trial. There's some [lack of clarity] about the security services' role in the investigation," she added, stressing that the murder took place in an area of Moscow "with a very high level of security: cameras, police, military people."

The Bolshoy Moskvoretsky Bridge is a stone's throw away from the Kremlin and Red Square.

"This is a little suspicious and I think there should be transparency about any kind of evidence and there hasn't been," she added.

'Responsibility' lies with Putin

Her report noted that the murder took place "in a context of an increasingly repressive climate in Russia" and that the number of attacks against opposition figures in the country has increased since then.

Vladimir Kara-Murza, a Russian opposition politician and chairman of the Boris Nemtsov Foundation, told Euronews that "these risks are so real, so tangible, we live with them."

He alleges that he has survived two assassination attempts, in May 2015 and February 2017.

"Both times I went to the hospital with severe poisoning, which induced coma and multiple organ failures," he said.

For him, Nemtsov was "absolutely irreplaceable for Russia, for Russian politics" because of his ability to communicate with people and draw them to the streets in their thousands.

"The political responsibility for the murder of Boris Nemtsov is borne by the Russian authorities and personally by Russian President Vladimir Putin," he said.

Square Boris Nemtsov

The furore over the lack of progress has yet to die down. Every year, marches are held in Moscow to commemorate the late politician. One is to be held on Saturday.

Prague in the Czech Republic, Vilnius in Lithuania and Washington D.C in the US have all inaugurated street signs bearing Nemtsov's name near Russian embassies.

Nemtsov's daughter, Zhanna Nemtsova, spoke on Thursday at the unveiling of the plaque in Prague.

"Boris Nemtsov was a representative of humanism in Russian politics. We had very few of them in history, unlike many tyrants," she said.