Life is cheap in Russia
In his last tweet, Mr Nemtsov sent out an appeal for Russia's divided opposition to unite at an anti-war march he was planning for Sunday.
"If you support stopping Russia's war with Ukraine, if you support stopping Putin's aggression, come to the Spring March in Maryino on 1 March," he wrote.
Speaking earlier this month to Russia's Sobesednik news website, he had spoken of his fears for his own life.
"I'm afraid Putin will kill me," he said in the article (in Russian) on 10 February.
Violent deaths of Putin opponents
April 2003 - Liberal politician Sergey Yushenkov assassinated near his Moscow home
July 2003 - Investigative journalist Yuri Shchekochikhin died after 16-day mysterious illness
July 2004 - Forbes magazine Russian editor Paul Klebnikov shot from moving car on Moscow street, died later in hospital
October 2006 - Investigative journalistAnna Politkovskaya shot dead outside her Moscow apartment
November 2006 - Former Russian spyAlexander Litvinenko died nearly three weeks after drinking tea laced with polonium in London hotel
March 2013 - Boris Berezovsky, former Kremlin power broker turned Putin critic, found dead in his UK home