Thousands of people gathered in central Moscow Sunday for a march to mourn murdered opposition leader Boris Nemtsov.
Russian authorities, usually slow to give permission for large opposition rallies, gave quick approval for Sunday's memorial march.
Nemtsov, 55, was to have led an anti-war and anti-Putin rally in Moscow. Instead, as many as 50,000 people are expected to come out to pay tribute to the man whom U.S. President Barack Obama called a "tireless advocate" for the rights of the Russian people.
Police are hunting for those responsible for shooting Nemtsov four times from a car while he and a female companion crossed a bridge over the Moscow River Friday night, just steps from the Kremlin. The woman was not hurt.
People have continued to leave flowers at the site where Nemtsov was killed.
Putin to oversee investigation
Russian President Vladimir Putin said the shooting looks like a contract killing, and he called it a "provocation." The Kremlin said he will personally oversee the investigation.
Putin sent a condolence letter to Nemtsov's mother, Dina Eidman, saying he sincerely shares her sorrow, and calling Nemtsov's death an irreparable loss.
But just a few weeks ago, Nemtsov told the Russian news website Sobesednik he thought Putin wanted him dead, and he did not hold back his contempt for the Russian leader.
"I'm afraid Putin will kill me. I believe that he was the one who unleashed the war in Ukraine. I couldn't dislike him more," Nemtsov said.
Ukrainian President Petro Poroshenko condemned Nemtsov's murder, calling him a "bridge" between Ukraine and Russia.
Obama called the killing "brutal" murder, and said Russia must carry out a prompt and impartial investigation.
France's President Francois Hollande expressed anger at Nemtsov's death. He called the shooting a "hateful murder," and described Nemtsov as a "defender of democracy."
Attempt at destabilization
Russia's Investigative Committee commented that it is possible the crime could be an attempt to destabilize the political situation. Spokesman Vladimir Markin said Saturday it also could be linked to Islamic extremism or the situation in Ukraine.
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, told Reuters of Nemtsov's murder.
Some media outlets blamed the murder on nationalists, who have been accused of several assassinations, including that of human rights lawyer Stanislav Markelov in 2009.
“I flatly deny any involvement of nationalists in these events,” Dmitry Dyomushkin, a leader of “The Russians” nationalist organization, told Reuters.
Nemtsov was a deputy prime minister in the 1990s, and many Russian observers predicted he would succeed then-President Boris Yeltsin.
After Yeltsin chose Putin as his successor, and Putin's subsequent election in 2000, Nemtsov became one of Russia's sharpest and most outspoken Putin critics, especially since last year's uprising in Ukraine.
In September, Nemtsov told VOA that Putin wants revenge for Ukraine's overthrow of its pro-Russian president.
He said Putin fears that what happened in Ukraine could happen in Russia, and sees a pro-European Ukraine as a threat to his own power.
Vladimir Markin of the Russian Investigative Committee said, "Currently the investigation is looking into several lines of inquiry. First of all of course, it is the possibility that the murder could be a provocation to destabilize the political situation in Russia. And Nemtsov could become a sacrificial victim for those who would not stop before using any means to reach their political goals."
"Also we are closely looking into a possibility that the murder could have links with Islamist extremism. The investigation has information that Nemtsov received threats linked to his position about the shooting at the Charlie Hebdo magazine office in Paris," Markin added.
"Besides, we are already checking a line of inquiry linked to the events inside Ukraine. It is no secret that on both conflicting sides there are very radical individuals not controlled by any authorities," he said.
Some material for this report came from Reuters.