BERKELEY — They wear black pants and sweatshirts, with either helmets or hoods over their heads, bandanas across their faces — and dark sunglasses, goggles or gas masks over their eyes. Many carry makeshift shields and flags, whose staffs can quickly become weapons.

They call themselves “antifa,” short for anti-fascist, and they’re part of a loosely organized national network of anonymous anarchists. The movement during much of this decade has been a common sight at Bay Area protests spurred by police shootings, the Occupy Wall Street movement and Donald Trump’s election as president.

But in the three weeks since these shadowy activists forcefully confronted neo-Nazis and white supremacists at a rally in Charlottesville, Virginia — where one counterprotester was killed, allegedly by a white supremacist — antifa has suddenly become a household word. Its members are the target of fierce anger on the political right whose members believe antifa’s penchant for violence is being excused while far-right mayhem is condemned. It’s also created a fiery debate among protest leaders on the left.

Some politicians and experts see antifa as vigilantes or even terrorists. Berkeley Mayor Jesse Arreguín last week suggested that they should be classified as a criminal street gang.

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“It’s glorified mob violence tied to an ideology,” Brian Levin, a criminology professor at Cal State San Bernardino who studies hate groups, said of antifa’s tactics.

The vast majority of the 4,000-some protesters who descended on Berkeley’s Civic Center Park last Sunday to demonstrate against a small group of Trump supporters were perfectly peaceful, but some of the 100 to 200 black-clad antifa there ganged up on the Trump fans, punching and kicking them.

Other antifa carried colorful shields painted with the words “no hate” to build a “barrier” that antifa claim is needed to protect anti-racist protesters from the police and right-wingers.Thirteen people were arrested at the Berkeley protests, and two were sent to the hospital.

Antifa members have been reluctant to talk to the media, but one member active in the Bay Area movement who was at the Berkeley protests spoke to this news organization on the condition that neither their name nor gender be disclosed.

The activist said antifa take to the streets “out of love,” keeping nonviolent protesters safe from right-wing activists and the police.

The group operates in small cells, the activist said. A few people might work together to plan a protest at far-right events, and members also monitor message boards and social media of members of organized far-right groups like the Proud Boys, Patriot Prayer and Oath Keepers, the activist said. Antifa’s members communicate over encrypted messaging apps like Signal.

Violence is justified, the Bay Area antifa member said, because the far right is attempting to create a fascist state.

“If the police try to attack protesters, antifa gives other people space to stay safe,” the member said, adding that even the threat of violence can be enough to dissuade right-wingers from attacking counterprotesters. “If you have a cocked fist, you don’t have to throw it,” the activist said, quoting Malcolm X.

The activist said antifa members don’t see a difference between neo-Nazis and the Ku Klux Klan and Trump supporters because the latter enable white nationalists and far-right activists. Those groups, the antifa member argued, “need to be answered in terms they can understand.”

Today’s antifa can be traced back to punk activists of the Anti-Racist Action movement in the 1980s and is modeled after earlier anti-fascist groups in Europe, according to Mark Bray, a lecturer at Dartmouth University who recently published a book on the movement, “Antifa: The Anti-Fascist Handbook.”

Members of Black Bloc — another term for the anarchist activists who make up antifa — have gone to battle over the issues of the day, from environmental issues to the economic inequity that sparked the Occupy movement, particularly in Oakland, Bray said.

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“What really brought it back in a sense and into view for the first time was the exposure that the alt-right received through the Trump campaign,” Bray said. Specifically, he said, it was the protests surrounding Trump supporter and right-wing provocateur Milo Yiannopoulos’ visit to Berkeley in February that put antifa “on the national stage and made it something that people tried to understand.” Yiannopoulos’ speech was canceled as antifa violence spread across the city.

Yiannopoulos is scheduled to return to Berkeley later this month with right-wing commentator Ann Coulter and, reportedly, former Trump adviser Steve Bannon. Antifa is intent on shutting down that event.

Bray said members as a general principle operate only alongside people they know and trust, which gives them more cover if criminal acts are involved. But, he said, most preparation for protests is done behind the scenes when antifa members research right-wing groups.

“In some ways it’s like being a private investigator,” said Bray, who interviewed about 60 members for his book. Many view the work as a “second job,” he said.“It’s not something people enter into lightly, and that’s why it takes the form it does.”

Those on the receiving end see antifa differently.

Arthur Schaper, a Trump supporter from Torrance, said he was attacked by antifa activists during last week’s Berkeley protest. He said several masked antifa members cornered him, broke his glasses, stole his red Make America Great Again hat, ripped a Trump flag off of his shoulders, pepper sprayed him and covered him with glitter.

“These are a bunch of spoiled, entitled kids who just want to rage against something,” Schaper said. “They’re evil thugs. They’re not social justice warriors. There is nothing just about them.”

Schaper has a reputation for interrupting town hall meetings of Southern California Democrats and is involved with an anti-LGBT organization that the Southern Poverty Law Center has classified as a hate group, but he said he opposes white nationalism and that the violence at the Berkeley rally was unprovoked.

“I couldn’t believe this was happening to me,” said Schaper, who spent hours Sunday arguing with counterprotesters.

Many local politicians are also fed up with antifa. House Minority Leader Nancy Pelosi, D-San Francisco, released a statement last week denouncing the group. “The violent actions of people calling themselves antifa in Berkeley this weekend deserve unequivocal condemnation, and the perpetrators should be arrested and prosecuted,” she said.

Politico reported Friday that the FBI has been warning states and cities since early last year that antifa has become “increasingly confrontational and dangerous,” and the Department of Homeland Security has classified its violence as “domestic terrorist violence.”

The organization has provoked intense debate on the left. Some activists argue that antifa is distracting from the larger message they’re trying to send: condemnation of racism and the Trump administration. Others believe antifa has an important role to play defending protesters from attacks by right-wingers.

Mansour Id-Deen, the president of the Berkeley chapter of the NAACP, said antifa’s actions in Berkeley were “counter-productive.”

“The violence never works,” he said. “It gives the other group — white supremacists, Nazis, whatever they call themselves — a national platform and raises their profile higher than it should be.”

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On Fox News and other conservative media, some of the Trump supporters who got beat up by antifa have found themselves transformed from political nobodies to heroes of the right.

Other local protest leaders say antifa isn’t a serious problem. Michael McBride, pastor at The Way Christian Center in West Berkeley, who helped organize Sunday’s protests, said antifa helped protect protesters.

“I know they joined our march and protests and were very deferential to our leadership as they followed us down the street,” McBride said at a news conference last Monday.

Oakland lawyer and former mayoral candidate Dan Siegel, who helped organize the Berkeley protests and is now defending a former community college professor accused of hitting a right-wing protester with a bicycle lock at an April protest, said right-wingers antagonized protesters by making racist comments and jeers.

Violence is a legitimate response to neo-Nazi ideology, he said. “As a Jew, an untold number of my family were liquidated by these Nazis,” Siegel said. “How can you look at these people who think that Nazism and fascism are fine and hear them talking about liquidating Jews and African-Americans? These are people who want to kill us.”

The bottom line, said the Bay Area antifa member, is that not just Nazis should be worried about antifa, but also “Nazi sympathizers.”

His definition of a Nazi sympathizer: “anyone who feels bad when a Nazi gets beaten up.”