Earlier this month at the Defend DACA protest in downtown Oakland, Brass Liberation Orchestra marched among the crowd like pied pipers of the resistance, drawing passersby into the procession with their jovial horns. The band members took turns as conductors, using hand gestures to quiet the brass instruments so protesters could chant along to the drum line’s beat: “El pueblo únido jamás será vencido! / The people united will never be divided!”

People who happened to show up to the march with musical instruments — including one person with an Afro-Brazilian berimbau — spontaneously joined the Orchestra as it marched through the streets. This is how Brass Liberation Orchestra has acquired most of its 20-something members: Through serendipitous run-ins at actions. The sprawling, multi-generational brass band has drummed up energy at Bay Area protests since the early 2000s, and most of its musicians encountered Brass Liberation Orchestra at a march before deciding to pick up their instruments and join.

A full-time musician going only by the name Breakfast, with salt-and-pepper hair and a pink fedora, remembers attending an Occupy protest in San Francisco in 2012, saxophone in hand, when he spotted Brass Liberation Orchestra. When rumors circulated that police would raid the camp that night, protesters divided themselves into two groups: One to defend the camp and one that didn’t want to risk arrest. Brass Liberation Orchestra was in charge of leading the second group to safety by using their music to guide people away from the conflict zone.

“Wow, so all these musicians have shown up and they have a functional role,” recalls Breakfast when I meet him and several other members of Brass Liberation Orchestra at a dive bar in the Mission district, before they head out to busk on a recent Friday night. “It’s not for expression or entertainment or any of these other roles we normally assign music to. Its role was to protect people’s physical safety.”

Candy Michelle Smallwood, an eviction defense lawyer by day, first heard Brass Liberation Orchestra at an anti-eviction march two years ago when she worked at the Housing Rights Committee of San Francisco. “There was this really awesome march that started at Dolores Park and went to different houses where people were facing eviction. Through this whole march — which lasted about two hours — there was this band that was playing.”

Smallwood was so inspired she got her flute out of storage. Two years later, she’s still with the Brass Liberation Orchestra as an alto saxophonist. Like Smallwood and Breakfast, Brass Liberation Orchestra members come from diverse professions and musical backgrounds: Some are rigorously trained musicians while others hadn’t played their instruments since the high school marching band. Sousaphone player Geoff Lee, for example, is a philosophy professor at UC Berkeley; his wife and fellow band member, drummer Deepa Varma, is the executive director of the San Francisco Tenants’ Union.

Brass Liberation Orchestra’s musical selections, while fun and upbeat, are charged with political meaning: The band has several songs in its repertoire from Afrobeat pioneer and human rights activist Fela Kuti. Anthems of Latin American resistance, like Los Fabulosos Cadillacs’ “Matador,” are also on rotation, as well as familiar pop songs like Eurythmics’ “Sweet Dreams.” In keeping with the band’s anti-capitalist spirit, Brass Liberation Orchestra’s sheet music is available on its website for free.

Brass Liberation Orchestra’s membership often increases parallel to major political events: The group first gained momentum around the anti-Iraq war protests of the early- and mid-2000s; many new members came along during the Occupy movement and, more recently, anti-Trump rallies. The band has performed at the 2014 Climate March in New York, workers’ strikes, racial justice rallies, and eviction blockades. Oftentimes, it uses its music for functional purposes: Guiding protest chants, disturbing the peace of predatory landlords and exploitative employers, drowning out white supremacists, and busking to raise money for victims of Hurricane Harvey and Charlottesville.

“At our best, we can diffuse tension, offer moral support, or bring joy to people risking arrest,” says drummer Joshua Cohen, who has a day job as a hospice social worker.

Brass Liberation Orchestra is part of a growing movement of radical brass bands that started in the late ’90s and early 2000s: Extra Action Marching Band, a like-minded group from Oakland, went on to tour with David Byrne. Cohen refers to New York’s Rude Mechanical Orchestra as Brass Liberation Orchestra’s sister band. Some former members of Brass Liberation Orchestra have also relocated to other states and founded similar bands in their cities, many of which meet up at HONK!, a festival for activist street bands that takes place in various cities around the country.

As we wrap up our interview, the members of Brass Liberation Orchestra pack up their saxophones and tubas to head to the corner of 18th and Valencia. Today, they’re busking to raise money for their trip to HONK! — a rare occasion, they tell me, where they collect money for themselves instead of a cause. I expect to see a few passersby drop dollar bills into an open saxophone case, but two songs in, a large crowd has already amassed. A woman shows up with a musical saw and joins the ensemble. Couples on dinner dates, families with strollers, and groups of friends getting ready to hit the town all cluster in close to avoid oncoming traffic as they watch the band.

As the band members catch their breath, trumpet player Jordan Baxter Stern introduces the next song, Nile Rogers’ “Do What You Wanna”: “We’re all about doing what you wanna unless you’re a rich asshole, in which case, you should give all your money away and do what other people tell you to do.”

Judging by their whoops and hollers, the crowd approves.