Western leaders are pressuring Moscow for a full and transparent investigation into the fatal shooting of opposition leader Boris Nemtsov, a staunch opponent of President Vladimir Putin.
Nemtsov, 55, a deputy prime minister in the 1990s who later organized mass rallies against Putin in 2011 and 2012. Most recently, he accused Putin allies of profiteering from the development of the Sochi Winter Olympics infrastructure.
President Obama on Friday described Nemtsov as "a tireless advocate for his country" and asked Russia to launch "a prompt, impartial and transparent investigation."
German Chancellor Angela Merkel's spokesman, Steffen Seibert, said she "appreciates the courage of the former deputy prime minister, who repeatedly expressed publicly his criticism of government policy" and "calls on ... Putin to ensure that the murder is cleared up and the perpetrators brought to justice."
But German Foreign Minister Frank-Walter Steinmeier went further, implying that Nemtsov's death was a political assassination, a view shared by other leaders and human rights groups.
Putin condemned the murder and promised to personally oversee the investigation.
Lithuanian President Dalia Grybauskaite tweeted: "[The] murder of Boris Nemtsov shows that Russia slides down to darkness of terror against its own people," Grybauskaite wrote on Twitter.
Amnesty International noted that the "cold-blooded murder" of Nemtsov must be added to an already long "list of unsolved political murders and attack in Russia, the investigations of which were under 'personal control' of senior Russian politicians."
"March of protest in Moscow also becomes march of mourning. Opposition and journalists must be protected!," Swedish Foreign Minister Margot Wallstrom tweeted, referring to an upcoming anti-government march being organized by Nemtsov and his allies.