MOSCOW — Immense, angry crowds turned out in Moscow Sunday to march through the city in honor of slain Kremlin critic Boris Nemtsov.
Nemtsov — an opposition politician and frequent critic of Russian President Vladimir Putin — was supposed to appear at a Sunday rally protesting Russia's role in the fighting in Ukraine. The rally was converted into a memorial march after the 55-year-old was gunned down late Friday on a street just steps away from the Kremlin.
More than 50,000 people marched along the banks of Moskva River under gray skies to the site of Nemtsov's death.
Many carried signs with a single word: "Fight" and clutched images of Nemtsov. Others held up posters that read, "No words," and "Propaganda kills." Nearly all chanted, "Russia without Putin" as they marched.
"It's us who are the true patriots," Nemtsov ally and former prime minister, Mikhail Kasyanov, told reporters at the march.
Nemtsov's death has ignited a fury among opposition figures, who have called the killing an assassination and admonished the Kremlin for creating an atmosphere of intolerance of dissents.
Nemtsov was working on a report confirming Russia's involvement in Ukraine's separatist movement at the time of his death, Ilya Yashin, a politician and activist who worked with Nemtsov, told NBC News. The papers have now been seized by investigators and may never see the light of day, Yashin said.
Nemtsov and his political allies were often vilified in the state media, where the opposition is dubbed "national-traitors" and "the fifth column," for their criticism of the increasingly authoritarian Russian leadership.
A spokesman for Putin called Nemtsov's killing a "provocation," and told Nemtsov's mother that the killers would be found and punished. The Russian president on Friday night ordered Russia's top law enforcement officers to investigate the killing, but no suspects have been identified. Investigators have listed a wide range of versions for Nemtsov's slaying, favoring especially the "sacral victim" theory, which accuses the opposition of killing one of their own to rouse anger toward the government.
But many of Nemtsov's supporters believe Putin was indirectly or directly responsible for his death. "The blood of Mr. Putin — the blood of Nemtsov on Putin's hands of course," because Putin has created an atmosphere of "hateness" and "non-tolerance," said one of the marchers Sunday.
"He was killed because he was against Putin's aggression in Ukraine," said marcher, 15-year-old Ivan Zakharov.
Ukrainian flags flew at the march, a rare sight in today's Moscow.
But Ukrainian lawmaker Alexei Goncharenko, who dared to attend the rally, was promptly arrested — despite his parliamentary immunity — on accusations of involvement in bloody clashes between the pro- and anti-Russian demonstrators in Ukraine's Odessa last May.
The opposition said Moscow city authorities had approved the march from 3 p.m. (7:00 a.m. ET), allowing for up to 50,000 people, though the organizers say more could show up to march alongside the River Moskva, according to Reuters. Other rallies in honor of Nemtsov took place in at least 15 other Russian cities.
This story originally appeared on NBC News.