Holding placards declaring "I am not afraid", thousands of Russians marched in Moscow on Sunday in memory of Kremlin critic Boris Nemtsov, whose slaying on the streets of the capital has shaken Russia's beleaguered opposition.
One organizer said that more than 70,000 people turned out in central Moscow on Sunday, many carrying Russian flags and slowly marching through an avenue alongside the River Moskva. Police estimated the crowd at more than 16,000. The Associated Press reported 30,000 attended the march.
Russian investigators say they are pursuing several lines of inquiry, including the possibility that Nemtsov, 55, was killed by Muslim attackers or that the opposition killed him to blacken the president's name.
Putin's opponents say such suggestions show the cynicism of Russia's leaders as they whip up nationalism, hatred and anti-Western hysteria to rally support for his policies on Ukraine and deflect blame for an economic crisis.
"It is a blow to Russia. If political views are punished this way, then this country simply has no future," Sergei Mitrokhin, an opposition leader, said of Nemtsov's murder.
Another mourning march was held earlier Sunday in St. Petersburg, drawing several thousand people.
Putin has described the killing as a "provocation," and told Nemtsov's 86-year-old mother, Dina Eidman, that the killers would be found and punished.
He also promised to do everything possible to bring to justice those responsible for Nemtsov's killing.
"Everything will be done so that the organisers and perpetrators of a vile and cynical murder get the punishment they deserve," Putin said in a telegram to Nemtsov's mother published on the Kremlin's website.
He said Nemtsov's death was an irreparable loss and that he had "left his trace in Russia's history, in politics and public life".
Nemtsov was one of the leading lights of an opposition struggling to revive its fortunes three years after mass rallies against Putin failed to prevent him from returning to the presidency after four years as prime minister.
Putin has been Russia's dominant leader since 2000, when ailing President Boris Yeltsin chose the former KGB spy as his successor, a role Nemtsov had once been destined to play.
Al Jazeera and wire services