Boris Nemtsov had, at one time, been tipped for the top job in Russia, but his political ambitions changed when he began to challenge the Kremlin.
He emerged as a leading figure in an opposition movement seeking to revive its fortunes.
But he was killed two days before a rally he had been helping to organise against Russia's alleged involvement in Ukraine and a faltering economy.
Russian President Vladimir Putin condemned the shooting as a "provocation", and vowed to bring those responsible for a "vile and cynical murder" to justice.
The motive for the shooting is not clear. Critics say the murder was the result of a climate of fear in Russia, where opponents are considered by some to be traitors, working to destroy the country.
At a time of rising nationalism - is the Russian opposition being stifled and intimidated?
Presenter: Martine Dennis
Ben Judah - Associate Fellow at the European Council on Foreign Relations and author of the book "Fragile Empire: How Russia Fell In and Out of Love with Vladimir Putin."
Sergey Markov - Political Strategist and Public Spokesman for President Putin.
Martin McCauley - Russia Analyst and Historian, and author of "The Rise and Fall of the Soviet Union".

Source: Al Jazeera