It started as a parking lot party, more friends than strangers, all drinking beer and enjoying a cool night near the end of May. They were celebrating the opening of the community arts center WonderRoot, touring its fancy new digs and waiting in the breeze for the bands in the basement to turn on the amps.

It didn't take long for the drags of an old familiar tune to blow through the parking lot off Memorial Drive, no amplifier necessary. After that first brass bellow was a tap on a drum, the shuffle of movement, then a full marching band in black-and-purple stripes, ribbons and tassels. It wailed "Saint James Infirmary Blues" and poured from the basement led by a double-belled sousaphone painted like a fire-breathing dragon. Euphonium, trombone, alto sax, tenor sax, bass and snare drums, cymbals, tambourine, a whistle.

Curtis Compton / AJC (ENLARGE)

Dianna Settles plays drum as she marches throught Lenny's Bar with members of the Atlanta Sedition Orchestra in Atlanta.

• See and hear the band in this audio slideshow UPCOMING PERFORMANCES The Atlanta Sedition Orchestra's performances are sometimes unannounced and spontaneous. But look for the musicians at these events ... • Sunday: Concrete Pandamonium at Eyedrum, 290 Martin Luther King Junior Drive S.E. • July 6: Atlanta PRIDE Celebration, Atlanta Civic Center, 395 Piedmont Ave. N.E. • July 12: Reynoldstown Wheelbarrow Festival, corner of Gibson Street and Kirwood Avenue. TO JOIN Sedition Orchestra rehearses once per week and is always accepting new members. For more information, visit: www.myspace.com/atlantaseditionorchestra. LIVING Latest Headlines: Zoo raises $40K in campaign to keep pandas  Georgia Aquarium to welcome 10 millionth visitor  Aquarium adds 4 sandbar sharks to exhibit  Cellphones serve as '$300 baby rattle'  Athens ranked among best places to live  • More Living Stories • Living photo galleries • Georgia Aquarium news and photos • Zoo Atlanta news and photos

Conversations stopped. Rookie audience members raised their eyebrows; regulars at protests and benefits smiled. It took a few songs before they were officially introduced.

But the drums were labeled: ASO.

"Some people were just shocked," says Chris Appleton, a WonderRoot founder. "A marching band?"

Like every gig the Atlanta Sedition Orchestra plays, it wasn't just a party. It was a parade.

The band started in late 2007, rehearsing in living rooms and borrowed spaces with half a dozen musicians. Since then, its membership has grown to 30 or so, and its list of gigs includes a Cabbagetown benefit, feminist talk show on WRFG-FM (89.3), bicycle auction and a fund-raiser for Grannies for Peace.

With traditions borrowed from high school football fields, New Orleans jazz funerals and protests marches, it attracts musicians without a venue and activists without an organization. The sound is unpolished, but soulful and spirited. The band is political like Infernal Noise Brigade, a well-known, now-defunct Seattle marching band that formed to protest 1999 World Trade Organization meetings. But it's all ages, spontaneous, community-oriented, fun and local like the Seed & Feed Marching Abominable from Inman Park.

No musical experience necessary, purple-and-black "uniforms" requested, passion required.

"We all share a kind of joy in taking public spaces," says Rebecca Miriam, 25, a sousaphone player who helped to start the group after she moved to Atlanta in August 2007. "People come to the band because they're on the same page and want to use their talents."

In radical bands and dance teams across the country, those talents range from superior musicianship and composing skills to willingness to pick up a cowbell and make it rock. New York City has its Rude Mechanical Orchestra; Somerville, Mass., the Second Line Social Aid & Pleasure Society Brass Band; Greensboro, N.C., the Caka!ak Thunder; San Francisco Bay area, the Brass Liberation Orchestra.

These motley bands of amateur aficionados and musical newcomers formed around the country for the last decade, sometimes with other groups as inspiration, but just as often without it, says Kevin Leppmann, an organizer for HONK! Festival, an annual gathering of street bands to be held in October in Massachusetts. The surest way to get people behind a cause, he says, is to tell them to bring a tambourine or a horn.

"The whole idea is to have fun reclaiming our public spaces, reclaiming our connections to each other," says Leppmann, a trombone player. "We were surprised as anyone to find that [bands] keeps emerging and producing all over. Something about the present sociopolitical situation leads people to pick up and instrument and make some noise."

Miriam organized Rude Mechanical Orchestra's dance team before she moved to Reynoldstown with her partner, Phil Andrews, 29, a trombone player. His story is typical of the Sedition Orchestra: He loved playing music during high school in Pennsylvania, but didn't like the structure, so he gave up his horn to play in a punk band. When he found the marching band with "a punk rock sensibility" as an adult, his mom mailed his trombone.

The Sedition Orchestra recruits through word-of-mouth and Craigslist. Their ad is simple: "We perform at marches and fundraisers around town in support of local causes ... If you've just started playing, you play every day, or haven't picked up the thing in years, we invite you to come play with us."

Joining gives people a chance to say with a straight face that they play the washboard or the megaphone for the ASO, if not the Atlanta Symphony Orchestra.

The going rate for a performance: free.

Whichever members are available make up the band that day. They play in whatever space is available. That usually means playing as part of the crowd, like in the WonderRoot parking lot, where the band tangled itself in the bikers, drinkers and artists who'd moved outside. The band paused sometimes for a tune, took dramatic sideways steps forward, laid down on the ground and played in the air. Then they'd spring up and dart to another side of their crowded, asphalt stage.

"It's challenging people to move around," says Appleton, the WonderRoot founder. "As soon as people got comfortable and got in their little spot to watch the band, they would move. That was a nice parallel with [Sedition Orchestra's] purpose."

That purpose varies member to member.

• Alessandro Chapman, 20, who borrowed a pair of cymbals so he could be involved: He wants to see the end of capitalism in the United States, and says the band can be a part of it.

• Hiron Roy, 22, a Georgia Tech student and alto sax player: He wants a group that keeps him playing and performing.

• Amy Plasman, 28, a euphonium player of eight months: She wants a different level of political involvement. "I don't usually just go to a march or a protest, but I will go with the band," she says.

Principles and purpose aside, they all agree with the good, old-fashioned idea that everything, from a sporting event to a protest, is more exciting when a marching band shows up.

The repertoire is a bit of a surprise too, ranging from "We Shall Overcome" to Salt-n-Pepa's "Push it."

Miriam and Andrews brought the idea for a band to Atlanta, but they moved back to New York this month. They'll tour with Rude Mechanical Orchestra this summer. Sedition Orchestra members pout about how much the organizers will be missed, but say they're not nervous about what happens next.

They make decisions together, sticking to what might be rules, if this were a rule-making kind of organization: Everyone in the band is an individual, everyone is there because he or she supports the cause (whatever it is that day) and everyone in the band matters.

They democratized quickly, finding others to lead rehearsals and create new pieces for the band. They want more members, a fuller sound, to see some faces from the crowd at rehearsals, a color guard to fly the purple and black.

There's no shortage of rallies and fund-raisers that could use a boost. Their services are in demand.

It's a parade. They just try to keep moving.