Left and right protesters gather in Berkeley for demonstrations

A counterprotester is stopped by Berkeley police just north of a gathering of the Proud Boys, a conservative group. A counterprotester is stopped by Berkeley police just north of a gathering of the Proud Boys, a conservative group. Photo: Scott Strazzante / The Chronicle Buy photo Photo: Scott Strazzante / The Chronicle Image 1 of / 18 Caption Close Left and right protesters gather in Berkeley for demonstrations 1 / 18 Back to Gallery

Downtown Berkeley on Sunday became, once again, the site of a conflict between right-wing protesters and counterprotesters, crushing the hopes of city officials for a mellow day.

For most of the afternoon, flocks of protesters surged along Berkeley’s central streets, defying police officers’ attempts to corral them. Meanwhile, in Martin Luther King Jr. Civic Center Park, a small group of “anti-Marxists” sparred with those who came out to denounce them as white supremacists.

The afternoon ended with more than a dozen arrests, several small fires being set and a lot of screaming and slogan chanting — but by evening, the commotion was over.

The confrontation stemmed from an announcement earlier in the week from the Proud Boys and another right-wing group that they might stage a gathering Sunday at Civic Center Park, the site of previous clashes.

A Proud Boys posting on Twitter indicated that the gathering was canceled, but nonetheless, approximately 400 counterprotesters gathered in Ohlone Park at 11 a.m. and marched along downtown Berkeley streets, heading toward the location of the originally proposed rally.

Some of the marchers were dressed in black, the common uniform of antifa (anti-fascist) activists. They held up signs such as “Hatefully targeting marginalized groups is not political discourse” and chanted slogans in support of a host of leftist political positions.

“We want to show them that hatred is not welcome here,” said Levon Johnson of Berkeley, who came to protest right-wing groups.

The Berkeley Police Department had prepared for possible violence by setting up barriers in Civic Center Park and by closing surrounding streets to vehicle traffic. The city also banned items that could be used as weapons, as well as face-covering masks, from the protest.

“We’re going to do what we can to keep people safe,” said Matthai Chakko, a city spokesman. Neither the conservative nor the opposing groups had obtained city permits.

By 12:30 p.m., several dozen right-wing demonstrators amassed at the marchers’ intended destination, wearing “Make America Great” caps and waving signs with slogans such as “No to Marxism in America” and “Slave labor made America great.”

With the group was Eddy Brock, 30, who said his parents were both immigrants from socialist countries, and he was there to protest communism and Marxism. He pointed at the police barriers, saying, “All these barriers aren’t for the 30 to 40 patriots who showed up.”

Arguments between the sides, not surprisingly, grew heated. Counterprotesters surrounded right-wing activists chanting, “Nazi scum, we’ll shut you down, Berkeley is an immigrant town.” Film crews from the conservative media outlet InfoWars, flanked by bodyguards, prowled around the park in search of controversy.

Around 1:45, a series of explosions erupted near the intersection of Center Street and Shattuck Avenue, several blocks from the park. According to Berkeley police spokesman Byron White, marchers threw fireworks at officers, who deployed handheld smoke canisters and fired rubber bullets in response.

In a statement Sunday evening, the department reported that officers arrested 20 protesters — mostly for possession of weapons in parks and on streets and sidewalks. Earlier in the day it posted photographs on its Twitter account of objects police confiscated, including rocks and hammers. They also reported that protesters started three dumpster fires, set one car on fire and vandalized an additional 20.

By 3 p.m., the marchers had largely dispersed and the smaller rally in Civic Center Park devolved into loud, but nonviolent, discussions between sides.

The Berkeley actions came on the heels of a rally organized by Patriot Prayer and the Proud Boys on Saturday in Portland, Ore. There, activists on the right squared off with counterprotesters, occasionally engaging in scuffles. Police arrested four people, and a reporter was injured covering the event.

Created by Vice Media co-founder Gavin McInnes in 2016, the Proud Boys adopt a somewhat coy relationship with white nationalism, saying they are “Western culture chauvinists” rather than racist but sharing members and actions with some white supremacist organizations. Its initiation rites include “Fight Club”-style beatings, and McInnes reportedly endorses violence as a political tactic. The Southern Poverty Law Center classifies the Proud Boys as a hate group.

Michael Cabanatuan and Jonathan Kauffman are San Francisco Chronicle staff writers. Email: mcabanatuan@sfchronicle.com, jkauffman@sfchronicle.com Twitter: @ctuan, @jonkauffman