Boris Nemtsov, former deputy Prime Minister of Russia, and one of Vladimir Putin's harshest critics, was shot dead in Moscow on Friday evening by an unnamed assassin. The politician, whose death has been described by David Cameron as a callous murder, was due to lead a major opposition rally in Moscow on Sunday against the war in Ukraine.

According to the BBC, Nemtsov was shot four times in the back with a pistol from a passing white car, which subsequently fled the scene. Flowers have been left at the site of the shooting.

Russia's investigative committee has been looking into various motives for the shooting, including the possibility that it was an attempt to shake up Russia's political field, and have suggested that radical Islamists could be behind the attack.

The former politician had criticised the corruption of Putin's regime in the past, as well as the recent war in Ukraine. Speaking in an interview earlier in February with a Russian news website, he spoke of his fears that Putin would try to kill him.

Garry Kasparov, a Russian pro-democracy activist and former world chess champion released the following statement on his Facebook page:

"Devastated to hear of the cold-blooded murder of my long-time opposition colleague Boris Nemtsov in central Moscow, quite close to the Kremlin. Shot four times, once for each child he leaves behind. A man of Boris's quality no longer fits Putin's Russia. He always believed Russia could change from the inside and without violence; after 2012 I disagreed with this.

When we argued, Boris would tell me I was too hasty, and that in Russia you had to live a long time to see change. Now he'll never see it. Rest In Peace."

Meanwhile the US President Barack Obama has also condemned the murder, calling on the Russian government to conduct a "prompt, impartial and transparent investigation".

On Saturday, opposition leaders were calling to cancel the opposition rally and hold a memorial event for Nemtsov instead, however the authorities have stated that this will not be permitted. In the meantime, investigations continue.