The Mann Center for the Performing Arts opened its season Saturday afternoon not in sylvan Fairmount Park but amid the golden glow of Mother Bethel AME Church's stained glass, its audience in the fervent communion of common purpose. Baltimore and social justice were on everyone's lips, even if nothing so specific could have been foreseen when plans for the concert were first laid.

It was the kickoff of the Mann's Liberty Unplugged! festival, the music center's months-long focus on Frederick Douglass, the Rev. Dr. Martin Luther King Jr., and Nelson Mandela, and so it was. But social justice being the unfinished business it is, by the time these musical performances and poetry readings reached the stage, they had gathered a new, grievous urgency.

There's an enervating contradiction built into gatherings like these. Pockets of society in need of a primer on humanity don't generally turn out for concerts about social justice. But as the parents of Matthew Shepard say, though you may sometimes find yourself preaching to the choir, even the choir needs to rehearse.

If it was a rehearsal, it was also an ambitious convening. WURD-AM (900) personality Nick Taliaferro was master of ceremonies, and Mark K. Tyler, pastor of Mother Bethel, was host of the event, dubbed Three Men, Three Movements.

"Unplug the hate, because love is here," chanted Greg Corbin, founder of Philly Youth Poetry Movement, one of several writers whose pastiches became meditations on civil rights. A panel discussion probed the audience's thoughts, and a video booth in the narthex stood ready to gather more from interviewees.

Play On, Philly!, the West Philadelphia after-school music program, brought a newly commissioned piece to the church in Society Hill. Hallelujahs of the Free by David Carpenter - a Philadelphia composer trained at Bates, Baltimore's Peabody Conservatory, and Temple University - scored his work for chamber orchestra and texts by Douglass, including "What to the American slave is your Fourth of July . . .". Some of the musical demands may have been a reach for the fifth to eighth graders, conducted by Play On founder Stanford Thompson. But as an experience for getting certain sounds into their ears and under their fingers, it undoubtedly served as a stepping-stone to Copland's Lincoln Portrait.

Carpenter has a nice sense of this vernacular - a quiet trumpet-and-percussion moment that stirs the piece to life, menacing drums that underscore the point of the celebration being a "sham," a skillful weaving in of the national anthem.

The afternoon's "three movements" were structured by festival artistic director Nolan Williams Jr., and each explored a theme - freedom from slavery, human rights, and voting rights. But there was also a surprise: a pop-up concert-within-a-concert. First, a lone audience member stood and sang, then another joined her, until there was an entire chorus repeating the eight-bar refrain: "We who believe in freedom will not rest until it comes."

From the foot-stomping, applause, and cheers it drew, you might have thought freedom, indeed, had.