Five men, all Chechen, went on trial in the Russian capital, Moscow, on Monday, accused of participating in the killing of Russian opposition leader Boris Nemtsov, who was gunned down in February 2015 on a bridge near the Kremlin in a murder that shocked the country.

The defendants face charges ranging from murder and illegally supplying a firearm to involvement in a contract killing by an organized group.

All five pleaded not guilty to the charges.

The alleged triggerman, Zaur Dadayev, served as an officer in the security forces of the Moscow-backed Chechen leader, Ramzan Kadyrov. Shortly after his arrest, Dadayev reportedly confessed to killing Nemtsov for insulting Islam and Kadyrov, but has since retracted his statement.

Putin critic

Nemtsov was killed aged 55, just days before he was expected to lead a major march by the Russian opposition and release a report focused on the involvement of the Russian military in the Ukraine conflict.

Many opposition activists in Russia remain unconvinced by the official probe into the killing, among other things condemning its failure to find out who was behind it.

Nemtsov, a former deputy prime minister under President Boris Yeltsin, was an outspoken critic of current President Vladimir Putin. He was particularly strident in his condemnation of Russia's annexation of Crimea in March 2014.

tj/msh (dpa, AP)