Thousands Mourn Boris Nemtsov at Somber March in Moscow

Holding flowers and carrying signs, tens of thousands of mourners marched Sunday through Moscow in memory of slain opposition leader Boris Nemtsov, an outspoken critic of Vladimir Putin who was gunned down yards away from the Kremlin by an unknown assassin on Friday night.

"Fascism has killed a person who was against it," marcher Vyacheslav Buralnik, who knew Nemtsov from the 2000s when he was a district director for Nemtsov's Union of Right Forces party, told VICE News. "They killed a fighter for the truth."

The mood was somber as the huge crowd shuffled along under a gray sky and a bone-chilling drizzle. People spoke in hushed tones and carried flowers bunched in even numbers, a sign of mourning in Russia. Some carried Russian flags with black ribbons and small white signs that read, "I am Boris Nemtsov," and "I have no words."

Buralnik carried sheets of phrases he wrote about his fallen comrade, including one that said, "If the person is gone, so is the problem. But you can't kill all of us!"

Although few blame the Kremlin for directly ordering a hit on Nemtsov, several opposition leaders have said that the patriotic paranoia whipped up by state-controlled media over the past year likely inspired his killing.

Related: Russian opposition blames Kremlin propaganda for killing Boris Nemtsov.

The "anti-crisis march" that Nemtsov and others had planned for Sunday wasn't expected to draw a huge crowd because city hall had ordered it to be held in a sleepy neighborhood far from downtown Moscow. But after four bullets to the back killed Nemtsov, activists canceled that march and planned a memorial procession past the Bolshoi Moskvoretsky Bridge where he was shot.

Massive turnout in Moscow for opposition rally in honor of slain anti-Kremlin politician Nemtsov (via — Paul Sonne (@PaulSonne)March 1, 2015

After initially refusing their request, city officials approved the new event, and a huge crowd turned out. The Moscow police, who are notorious for under-estimating the attendance at street protests, said 21,000 people had marched, but some estimates claimed the actual number was as high as 70,000. It was likely the largest opposition march since 2012, when investigators opened criminal cases against more than two dozen protestors following clashes with police on the eve of Vladimir Putin's re-inauguration.

Nemtsov came to nationwide prominence as first deputy prime minister in the 1990s and was widely expected to become president after Boris Yeltsin, who eventually picked Putin as his successor. Instead, Nemtsov became a leader of the liberal pro-Western opposition, publishing several reports on state corruption, including one on the Sochi Winter Olympics.

Related: Russia awards lucrative Crimea bridge contract to Putin's crony. 

Putin's spokesman has said Nemtsov was killed as a "provocation" against Russia, while the country's powerful investigative committee has said it suspects Nemtsov was killed by Islamic terrorists for supporting the Charlie Hebdo journalists shot in Paris.

But prominent members of the Russian opposition have argued that Nemtsov's death is linked to the paranoia that state-controlled television has drummed up over the Ukraine crisis, with Putin himself warning that "fifth-columnists" are attempting to undermine Russia. Dmitry Gudkov, the only remaining Putin critic in parliament, told VICE News on Saturday that state media's "machine of hatred" was likely to blame for the opposition leader's killing.

"The atmosphere is bad here. We have too many fascist-style warriors," Buralnik said, citing members of pro-Kremlin youth groups and the traditional Cossack military caste, many of whom have joined Kremlin-backed rebels in eastern Ukraine. "They call anyone who lays flowers here 'fifth-columnists.' Now there are no leaders left, and in an empty political field, it's not flowers that grow."

Related: Russian newspaper to release document that allegedly links Kremlin to Ukraine invasion. 

Besides the many middle-class supporters of Russia's "democratic opposition," the march for Nemtsov also drew several groups of nationalists, some of whom held Russia's white, yellow, and black imperial tricolor. One of them said that, although they didn't agree with Nemtsov's liberal, pro-Western political views, they respected that "he wasn't afraid to oppose the regime."

The lawyers of imprisoned Ukrainian pilot Nadya Savchenko, who has been on a hunger strike in a Moscow detention center for more than two months, also took part in the march and called for her release. Several protestors carried Ukrainian flags to commemorate Nemtsov's outspoken opposition to the Russia-backed conflict in eastern Ukraine. According to fellow opposition leader Ilya Yashin, Nemtsov had been preparing a report about the flow of Russian weapons into eastern Ukraine.

Nemtsov's girlfriend Anna Duritskaya, who was walking with him when he was shot, is Ukrainian. The investigative committee has even said his killing could be connected with "internal Ukrainian events," suggesting Kiev assassinated one of its biggest supporters in Russia in some sort of nefarious plot to discredit the Kremlin.

Activist Leonid Volkov, who helped organize the march, told VICE News that the event could be a chance for the opposition to "get united again," and show it was not afraid to come out despite Nemtsov's murder.

Related: There's no saving Russia's imploding economy from the world's hurt feelings. 

But many other marchers doubted that the event marked the dawn of a new wave of opposition protests like those that riled the Kremlin in 2011 and 2012. Instead, they predicted a darker period of economic unrest.

Psychologist Alexander Venger, who was holding a printout of Pablo Picasso's Guernica emblazoned with the words "Murderers! You can't scare us!" argued that the economic woes caused by low oil prices and Western sanctions would soon provoke anger against the regime from people outside Moscow's mostly middle-class opposition.

"In the future, the activity won't be from us but rather from the hungry population," Venger said. "I think there will be political changes, but not in the direction we want."

Follow Alec Luhn on Twitter: @ASLuhn