Bulletproof vest. Check. Arm and kneepads. Check. Two-way radio. Check.

John Cookenboo, 27, knows exactly what he’s packing Saturday when he shows up to demonstrate against a group of right-wing activists in San Francisco — wherever they choose to appear — and at a similar gathering in Berkeley on Sunday.

The Oakland resident, who will be among hundreds if not thousands of counterprotesters expected to turn out for the latest mobilization of the far right under President Trump, brings his own brand of opposition. And it’s a far cry from the many peace vigils planned elsewhere in the Bay Area.

Cookenboo is part of what’s commonly called antifa and described as an extreme movement of its own, but on the left. He will join an unknown number of people seeking to directly confront those behind Saturday’s Patriot Prayer gathering, even if it means getting physical. He says he was ready when the event was initially planned for Crissy Field, and that if the group follows through with its last-minute decision to hold what it claims will be a “press conference” in Alamo Square instead, he’ll be there.

“I don’t have the widest build,” said Cookenboo, who took a few knocks from political opponents at a recent pro-Trump rally in Berkeley and ended up spending four days in jail. “But I’m tall, and I know my limitations. I know how to get done what I need to get done.”

Cookenboo said he speaks only for himself when he explains his goal this weekend is to make sure those who espouse hateful ideology feel unwelcome, even fearful, in the Bay Area.

The organizers of Saturday’s San Francisco event insist it isn’t open to racists or bigots, like those who turned out for the march in Charlottesville, Va., on Aug. 12. A car allegedly driven by a neo-Nazirammed a crowd of counterprotesters at that rally, killing a woman and injuring 19 people.

However, some of Patriot Prayer’s rallies have drawn white nationalists and militia groups.

“The alt-right has a tendency to wear a lot of military gear,” Cookenboo said. “When you see them come out like that, it’s definitely intimidating. You have to have people on the other side to provide a counterbalance.”

The loosely connected antifa coalition that has come to include disgruntled Democrats, radical leftists and anarchists has no organized structure or strategy. Its common target, however, has become groups it sees as racist and emboldened by the Trump presidency.

Antifa made headlines in Berkeley this year when protesters, often wearing masks and dressed in black, violently clashed with Trump supporters at right-wing rallies. The protesters smashed windows and set fires in UC Berkeley’s Sproul Plaza outside a hall where conservative commentator Milo Yiannopoulos was scheduled to speak in February. And they were at the center of clashes with Trump supporters in March and April at the same downtown Berkeley park where another right-wing rally is scheduled for Sunday.

Their tactics have antagonized many on the left as well as on the right.

“The antifa is incredibly disruptive and not worthy of the attention,” said San Francisco resident Buck Bagot, 66, a longtime community organizer for progressive causes. “In some ways, they’re taking the easy, juvenile way out.”

Their violence, Bagot said, doesn’t help with the difficult task of turning liberal goals into policy. When he was organizing for the recent Occupy movement and fighting for economic equality, Bagot said, similar tactics by masked “black bloc” elements helped turn the public against the cause.

But Cookenboo, who also was a fixture at Occupy demonstrations, said the importance of a strong resistance, and the media attention it generates, can’t be understated.

“It’s empowering,” he said.

The East Bay native said he never had any intention of coming to blows with political foes earlier this year when he attended the right-wing rallies in Berkeley. Still, in March, he went home with a welt on his head after he said he helped fellow activists injured in fights escape to safety.

In April, he was arrested on suspicion of carrying a knife and wearing a mask before a Berkeley rally started. Cookenboo said he forgot the knife was in his bag, and prosecutors have not charged him.

“Every time I’ve been arrested or detained, it’s been only for a short period,” he said, noting his connection to the National Lawyers Guild, a group of attorneys readily available for activists on the left. “All of the risks, they’re worth it.”

While many of his cohorts seek to hide their identity, Cookenboo does not. He said he’s well-positioned to weather any repercussions his activism might cause: He’s single, with no children, and his employer at a cannabis processing center in Oakland isn’t bothered by his political devotion.

“My name’s out there. I’ve already got the alt-right trolls giving me death threats,” he said.

Cookenboo said he and his partners also keep tabs on their opponents over the Web, but stop short of provoking them online.

On Friday, Cookenboo hadn’t finalized his plan for the weekend. But he said he expected to meet up in San Francisco and Berkeley with fellow activists. All will be similarly dressed in protective gear purchased at Army surplus stores or on eBay, he said.

Cookenboo’s crew knows what to expect. Their advance communication is being done through social media or texting, as most of the activists have been in touch — and demonstrated together — for years.

When they arrive, they’ll try to stick together, as they do at events where they’re seen in online videos as clusters in black or military fatigues near the right-wing demonstrators. They also use two-way radios and instant messaging services such as Signal. But if the protest is crowded or unruly, communication can be tough.

“Me and my friends are pretty good at watching each other’s backs,” Cookenboo said. “For the most part, it hasn’t failed me yet.”

Cookenboo said he hopes the weekend demonstrations won’t bring clashes.

“With all the media coverage, there’s going to be an outpouring of the public coming out to stand up against hate speech,” he said. “I don’t plan on doing anything impractical. I plan to react as needed.”

Kurtis Alexander is a San Francisco Chronicle staff writer. Email: kalexander@sfchronicle.com Twitter: @kurtisalexander