Thousands of people have marched through the Russian capital, at times chanting that President Vladimir Putin must resign. Police authorized the major demonstration in memory of Kremlin critic Boris Nemtsov.
Police estimated that more than 21,000 people marched through the Russian capital on Sunday to mourn Boris Nemtsov, shot dead on Moscow's streets late on Friday. A volunteer group charged with monitoring turnout, meanwhile, claimed that more than 50,000 had passed through metal detectors before the march.
"Vladimir Putin must resign," the crowd chanted as it edged along the Bolshoi Moskvoretsky bridge, the site of the shooting, next to the Kremlin. Many held flowers, waved Russian flags, or held pictures of the opposition politician and former deputy prime minister.
Some carried large banners of Nemtsov's face reading "Heroes Never Die," the same slogan used in Ukraine to honor more than 100 people killed in the public protests that toppled the former president, Viktor Yanukovych. Nemtsov had been critical of Russia's stance in the Ukraine conflict.
The former deputy prime minister, a sharp critic of President Putin since he took power in 2000, is the highest-profile politician to be assassinated in post-Soviet Russia. Amnesty International on Sunday called for the killing to be "meaningfully investigated."
"In the current climate of crackdown on freedom of expression, assembly and association, this is a cold-blooded murder of one of those free voices whom the authorities have so actively sought to silence," said Denis Kivosheev, Amnesty International's Regional Deputy Program Director for Europe and Central Asia, who expressed skepticism over Putin's pledge to personally oversee the investigation.
"There is already a list of unsolved political murders and attacks in Russia, the investigations of which were under 'personal control' of senior Russian politicians. We cannot allow Boris Nemtsov to become just another name on the list," Kivosheev said.
'I hope we won't get scared'
Russia's federal investigative agency said it was looking into several possible motives for the killing. Investigators have offered 3 million rubles (roughly 45,000 euros or $49,000) as a reward for information leading to the shooters. Officially, investigators have suggested Islamic extremism - Nemtsov was Jewish and criticized the attacks on "Charlie Hebdo" in France - and the possibility of an inside job by the opposition as potential motives, saying Nemtsov might have been a "sacrificial victim for those who do not shun any method for achieving their political goals."
Putin's spokesman Dmitry Peskov said that the presidency suspected the shooting to be a "provocation." Nemtsov's supporters instead blame the authorities for the murder.
"Essentially it is an act of terror. It is a political murder aimed at frightening the population, or part of the population that supported Nemtsov and did not agree with the government," Ilya Yashin, a friend and fellow opposition leader, told the Associated Press news agency on Sunday. "I hope we won't get scared, that we will continue what Boris was doing."
US Secretary of State John Kerry, appearing on ABC's "This Week" on Sunday, said that Washington had no concrete intelligence on who was behind the shooting.
"The bottom line is we hope there will be a thorough, transparent, real investigation, not just of who actually fired the shots, but who, if anyone, may have ordered or instructed or been behind this," Kerry said.