Russian President Vladimir Putin has said he will do everything possible to bring to justice those who committed the “vile and cynical” murder of opposition politician Boris Nemtsov.

Mr Nemtsov was shot four times in the back on a bridge near the Kremlin.

Western leaders demanded a transparent investigation into the killing.

In a telegram to Mr Nemtsov’s 86-year-old mother, Dina Eydman, Mr Putin said: “We will do everything to ensure that the perpetrators of this vile and cynical crime and those who stand behind them are properly punished.”

He said: “Please accept my deepest condolences in connection with this irreparable loss. I sincerely share your sorrow.

“Boris Nemtsov has left his mark in the history of Russia, in its political and public life. He occupied significant posts in a difficult time of transition in this country. He always openly and honestly voiced and upheld his views.”

Expressing shock at the “cruel and cynical murder”, Prime Minister Dmitry Medvedev said Mr Nemtsov was a “principled person” who “acted openly, consistently and never betrayed his views”.

He also offered condolences and praised Mr Nemtsov’s openness and honesty.

On Saturday there was a steady stream of people leaving flowers at the site of the killing.

Mr Nemtsov, 55, 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, particularly on the Ukraine conflict.

In a recent interview, Mr Nemtsov had said he feared Mr Putin would have him killed because of his opposition to the war.

Mr Nemtsov died hours after appealing for support for a march on Sunday in Moscow against the conflict.

The march, due to be held in a Moscow suburb, has now been cancelled, and the organisers have been given permission to hold a mourning procession in the centre of the city.