It is a truism that some of the most stirring and jubilant styles of music come from oppressed peoples - like Celtic, Cajun, blues and South African music, to name a few.

The music of South Africa's Ladysmith Black Mambazo definitely qualifies, given that the group hails from a country where the black majority was oppressed by the white minority for decades.

To most American audiences, Ladysmith Black Mambazo seemed to emerge, fully formed, out of nowhere when Paul Simon tapped the great a cappella vocal group to back him on his 1986 "Graceland" album and ensuing tour.

But the truth is, Mambazo - which comes to the Power Center tonight - has been weaving its rich, mercurial vocal harmonies and preaching the healing power of music for more than 30 years.

And even though the group's profile in the United States and Europe was raised considerably by that '86 tour, the group was already very popular in South Africa. In fact, the Mambazo sound was forged in the late '60s, when leader Joseph Shabalala wedded the intricate arrangements and harmonies of traditional Zulu music to the uplifting message of Christian gospel music. Shabalala also became a Christian in the '60s.

"We wanted to bring the gospel of loving one another all over the world," Shabalala is quoted as saying in the group's press materials, although he stressed that the music is not preaching any particular religious viewpoint. "Even without understanding the lyrics, this music gets into the blood. It evokes enthusiasm and excitement, regardless of what you follow spiritually."

Mambazo's latest release emphasizes that mission with its title - "Raise Your Spirit Higher - Wenyukela." The emotional ante is upped by the group's announcement that the album is dedicated to the 10-year anniversary of the fall of apartheid in South Africa.

Several songs explore spirituality, while others address the themes of cultural identity, struggle and cultural clashes in both South Africa and in other similarly divided countries. Although the songs are sung primarily in Zulu (with occasional English passages), the English translations of some of the titles underscore these messages: "Black is Beautiful," "Music Knows No Boundries," "Lord's Work," "Message From His Heart," "Lord is the Light and Truth," "Beautiful Wedding" and so on.

Shabalala is credited with developing the "isicathamiya" approach - in which the lead singer engages in a call-and-response "conversation" with the other singers, who use intricate, percussive mouth sounds like clicks, pops, tongue rolls and shouts. It's a difficult style to learn, and it requires extensive rehearsal to perfect. Shabalala said the style came to him in dreams, in which the singers appeared as angels.

Evy Warshawski, executive director of the Summer Festival, who calls herself "a huge a cappella fan," said that Mambazo's sound is "one of a kind - those harmonies are just so crystal clear, and their vocal arrangements are just so unique. I also like that their music played such an important part in the movement in South Africa; many of their songs have been about freedom. And they bring a sense of their community and lifestyle and their struggles to us in a very unique way."

But if Mambazo's musical ethic is to find affirmation in the face of adversity, the group members faced the most painful challenge of their lives in 2002 when Shabalala's wife, Nellie, was shot and killed by a unknown masked gunman in the parking lot of their church in South Africa. No arrest has been made, despite a long investigation.