More than 1,000 people were shouting “Shut it down” outside the University of California, Berkeley venue where Milo Yiannopoulos planned to speak on Wednesday when a group of black-clad, masked protesters carryings flags and shields arrived to put those words into action.

The band of about 150 anti-fascist activists – or antifas – quickly and efficiently stormed the multilayered police barricades that kept the crowd away from the entrances of the Martin Luther King Jr student union.

Sequined pink flags were revealed to be mounted on fireworks, which were launched at the building. Others smashed windows with the now vanquished police barricades. A portable police spotlight was toppled, graffitied and eventually torched.

In just 15 minutes, the speech by the infamous rightwing internet troll was cancelled. Love them or hate them, the antifas accomplished what they had set out to do.

The announcement of a university speaking engagement for a figure like Yiannopoulos might as well be the opening bars to a very familiar song: students express outrage, the speaker’s supporters cry censorship, and the institution invokes the liberal values that led the American Civil Liberties Union to sue on behalf of neo-Nazis’ right to march through a Jewish suburb.

The debate at Berkeley, home to the free speech movement of the 1960s, followed the same pattern, with the chancellor, Nicholas Dirks, defending the right to free expression, writing: “We cannot afford to undermine those rights, and feel a need to make a spirited defense of the principle of tolerance, even when it means we tolerate that which may appear to us as intolerant.”

But for a small but growing band of activists, any public appearance by a member of far-right, white-supremacist movements is an opportunity for a very different kind of response: resistance by any means, whether that means punching a neo-Nazi in the face or denying a member of the “alt-right” a platform.

“The rulebook has been thrown out,” said one of the protesters at Berkeley on Wednesday, who gave their name as Lana Wachowski, which is also the name of the writer and director of The Matrix. “It’s absolutely acceptable to use violence. They are 100% certain to use it against us.”

Black-clad antifa activists in Sacramento, California, made national headlines in June when they faced off against a white nationalist group, the Traditionalist Workers party, in a melee at the state capitol that sent 10 people to the hospital. Earlier this month, the group successfully shut down Yiannopoulos’s speech at the University of California Davis.

Jose, an antifa who was among them and asked not to be identified by his real name, said: “We get a lot of heat for physical confrontation but that’s the sort of language that is spoken by neo-Nazis,” he said. “That’s the only thing they understand.”

Donald Trump’s presidential campaign energized and reanimated various pockets of far-rightwing America, from the dregs of the Ku Klux Klan represented by David Duke to the seemingly ascendant “alt-right”, members of which gathered in Washington DC after the election at a “thinktank” conference that saw audience members participate in Nazi salutes.

Whether Trump’s authoritarian impulses and vehement xenophobia portend an era of American fascism has been the subject of academic and journalistic debate for well over a year, but for anti-fascists, this is no time for discussion.

“I’m pretty decided on the conclusion that it will be a total slide into fascism,” said Evan, an antifa from St Louis who asked not to be identified by his real name. “I think resistance is our only shot.”

Taking on a rightwing resurgence

Anti-fascist organizing has long existed outside of mainstream leftwing organizing in the United States. But as the far right has gained stature and attention amid the rise of Trump, anti-fascism has gained relevance.

“The resurgence of the far right really began in the wake of Black Lives Matter, especially Ferguson,” said James Anderson, the editor of itsgoingdown.org, a website that promotes anti-fascist organizing. “Trump was another shot in the arm for them.”

“Anti-fascism has really grown in response to the far right trying to get back into the streets,” Anderson added, comparing its growth to the “explosion of anti-capitalist, anti-authoritarian ideas” that occurred during the Occupy movement.

Shane Burley, a journalist and researcher who studies the far right, said that anti-fascists struggled to be “taken seriously” by other leftists in recent years, as mainstream groups took aim at “systemic racism” rather than specific racist groups.

But with the rise in “violent, casual racism” after the election, anti-fascism tactics will gain in popularity, he said.

Protesters demonstrate against a scheduled speaking appearance by Milo Yiannopoulos. Photograph: Ben Margot/AP

One popular tactic for antifas is simply gathering and publishing information about people involved in white nationalist groups.

“A lot of the work is to let the neighbors know who their neighbors are, if their neighbors are neo-Nazis,” said Michael Novick, an antifa activist from Los Angeles.

Novick’s group, Anti-Racist Action-Los Angeles, keeps close tabs on figures such as LA-based lawyer William Daniel Johnson, a white nationalist who was briefly named as one of Trump’s delegates to the Republican national convention in May.

NYC Antifascist Action said by email that “95% of what we do does not involve any kind of direct confrontation”.

Those activities include educational work, self-defense training and helping people who want to leave white-supremacist groups, they said.

But other antifa tactics can be at odds with the liberal ideals of non-violence and freedom of speech.

On 20 November, Evan and a group of about 30 other anti-fascists formed a “perimeter of defense” around a group of activists holding a Transgender Day of Remembrance event in a public garden. The ceremony proceeded without incident, but, Evan said, if anyone had attempted to harass the group, “they would have had to fight their way through”.

When Jose counter-protested the Traditional Workers party event in Sacramento, the goal was not to offer an alternative message to white nationalism, but to prevent the event from taking place at all.

“I’m Chicano. My family has dealt with racism, transphobia, homophobia, so these issues are not just words but violence,” he said. “Giving these people a platform is also giving them an opportunity for more violence. I don’t see what they’re doing as just speech. I see it as a violent act.”

I don’t see what they’re doing as just speech. I see it as a violent act Jose

While the violence in Sacramento was not the goal of the counter-protest, Jose said, it was an acceptable outcome.

“We never want anyone to get hurt, but our mission and our goal was to stop them from holding their event, and that happened,” Jose said.

‘We’re prepared to take whatever actions are necessary’

While antifas may be gaining visibility and influence, it remains to be seen whether their tactics can translate from shutting down marginal figures such as Yiannopolous to taking on the power of the US government – if their fears come true.

Evan’s group in St Louis is currently working on establishing a phone tree so that they can quickly respond to hate crimes or harassment.

“What we are ready to do essentially is be non-compliant,” said Zaina Alsous, an activist from Durham, North Carolina, where anti-fascist activists mobilized in December to counter a planned KKK rally.

That might include “frontline defense preparation”, Alsous said, such as “obstruct[ing] ICE buses” in the event of mass deportations.

“As for Trump, he will have to go, and ousting him and the agenda he will enact will require lots of different segments of society working together, and if not together, toward the same goal,” NYC Antifascist Action said. “Antifa will – with anarchists and other radicals – form the most militant end of this movement.”

“We’re prepared to take whatever actions are necessary.”