MOSCOW -- Tens of thousands of Russians have marched through central Moscow to mourn slain opposition politician Boris Nemtsov and express outrage at the Kremlin over his killing.
The march was organized by opponents of President Vladimir Putin.
Organizers of the event said about 70,000 people attended the march in Moscow. Russian police, who often give unrealistically low crowd-size estimates for opposition rallies, said only 21,000 people were involved.
The march began after thousands of people piled flowers on the bridge near the Kremlin where the opposition politician and former deputy prime minister was shot dead late on February 27.
The Bolshoi Moskvoretsky Bridge has become an impromptu monument for the slain opposition leader.
Standing on the bridge, opposition politician Mikhail Kasyanov -- who was prime minister during Putin's first term -- led the demonstrators in a chant of "We won't forget, we won't forgive."
One middle-aged woman told RFE/RL she believed "Putin was responsible" for Nemtsov's killing, even if it was not done under his orders.
Several marchers held signs with pictures of Nemtsov and the words: "These bullets hit every one of us."
"So much hate has been built up on this country that I suppose it was bound to happen," a young man told RFE/RL.
Several marchers held signs reading "Putin = War," and one woman wore a sign saying "Nemtsov = Love, Putin = War."
"I could not do otherwise," one elderly marcher told RFE/RL when asked why he came.
He said he hoped the rally would set change in motion but feared there would be no "breakthrough" or reform of the Russian political system following Nemtsov’s killing.
The state-run TASS news agency quoted police in St. Petersburg as saying about 6,000 joined a similar march there.
PHOTO GALLERY: Moscow memorial march
Putin has described the killing as a "provocation," and told Nemtsov's mother that the killers would be found and punished.
But Nemtsov had said in an interview that he feared Putin might want him dead because of his criticism of Russia's role in Ukraine's separatist conflict.
Russia's Investigative Committee says it is pursuing several lines of inquiry, including the possibility that Nemtsov may have been killed by radical Islamists as well as suggestions by Putin that his murder was a contract killing aimed at blackening the government's reputation.
Investigative Committee spokesman Vladimir Markin said Russian investigators had evidence that Nemtsov received threats "because of his position over the shooting of Charlie Hebdo staff in Paris."
Markin also said, "The murder could be a provocation to destabilize the political situation in the country."
He added, "Nemtsov could have been chosen as a sort of 'sacrificial victim' by those who don't hesitate to use any methods to reach their political goals."
Markin said a possible link to the conflict in Ukraine also was being investigated, as well as possible motives linked to Nemtsov's business interests or to his personal life.
Accusing The Kremlin
All five potential motives announced by the Investigative Committee deflect blame placed on Putin and his government by Kremlin critics who say the Kremlin was ultimately responsible.
In New York on March 1, Georgia's former President Mikheil Saakashvili said in an interview with Sky News that Putin "really turned Russia into some kind of a mafia clan and mafia state."
"Everybody that crossed Putin's way somehow ended quite badly, almost everybody -- Anna Politkovskaya, the most prominent Russian journalist; [Aleksandr] Litvinenko; Boris Berezovsky; certainly in Europe Putin most of all hated [former Polish] President [Lech] Kaczynski and he somehow also got killed in a plane crash, incidentally in Russia," Saakashvili said.
"I am not claiming that all of this was engineered or done personally by Putin. What I am saying is that it's a strange coincidence, a very weird one, that all of his main critics and everybody that crosses his way somehow almost all of them ended this way."
Anna Veduta, a former Russian opposition spokeswoman and a friend of Nemtsov, told CNN on March 1 that the circumstances of the slain opposition leaders' slaying were "convoluted."
"People like him are always under surveillance in Russia. In this sense he had [so-called] bodyguards, but they were from Russian state agencies. And this makes the circumstances of his murder even more convoluted because there is no way he wasn't under the state surveillance," Veduta said.
"Moreover, there were surveillance cameras all over the place because [the murder site] is only a few meters away from the Kremlin."
Ukrainian President Petro Poroshenko said on February 28 that Nemtsov told him about two weeks earlier that he planned to publish evidence of direct Russian military involvement in Ukraine's separatist conflict.
"Someone was very afraid of this.... They killed him," Poroshenko said in televised comments shown in Ukraine.
Kyiv and many in the West accuse Moscow of sending soldiers and arms to support separatist rebels who have risen up in eastern Ukraine, an accusation Russia has denied.
Nemtsov's lawyer told RFE/RL's Russian Service that Nemtsov was a victim of Russia's political regime no matter who pulled the trigger.
"The political responsibility for this murder lies with the authorities and personally President Putin -- those who started and are fighting a war and are running a propaganda campaign of hatred in its support," former liberal Yabloko party leader Grigory Yavlinsky wrote on Facebook.
Opposition leader Aleksei Navalny questioned how Nemtsov could have been killed, saying he would have been under surveillance ahead of the March 1 planned rally.
"I can't believe that yesterday night he could have strolled toward the Kremlin without being watched," he wrote on his blog.
Investigators said gunmen fired at least eight shots from a car as Nemtsov walked with a woman named by Russian media as 23-year-old Ukrainian model Anna Duritskaya, who was unhurt.
Footage aired by Russian TVC channel from a distant camera showed the suspected killer running along the road and jumping into a waiting car, which then sped off.
Nemtsov's killing has been condemned around the globe.
On February 28, UN Secretary-General Ban Ki-moon said he "was shocked by and condemns the brutal killing" of Nemtsov.
Amnesty International said his killing "must be promptly, impartially and effectively investigated."
The killing came just hours after a radio interview in which Nemtsov denounced Putin's "mad, aggressive" policies and the day before he was to help lead a rally protesting Russia's actions in the Ukraine crisis and the economic crisis at home.
After his death, organizers canceled the rally and instead called for a demonstration to mourn him on March 1 in central Moscow.
The city gave quick approval for that gathering, in contrast to its usual slow and grudging permission for opposition rallies.
The opposition leader's slaying adds to a growing list of Russian activists, journalists, and politicians who have challenged the Kremlin since the collapse of the Soviet Union some 25 years ago.
With reporting by Tom Balmforth in Moscow; additional reporting by Reuters, AP, AFP, and TASS