When does a string of gigs become a festival? At Friday's La Linea concert, the defining moment occurred at the end of Susana Baca's encore, when she invited previous performers Lila Downs and Yusa back on stage to dance. And when Downs's backing band danced in to bash extra percussion instruments, the joyful, chaotic unity made a fine end to a great evening.

Following Yusa's energetic opener, Downs rushed on stage in a blur of colourful clothes, black boots and unfeasibly long plaits. She dedicated La Nina "to all the working women of the world, in the hope that one day you will be equal to everyone else". Like many of her songs, it has a pretty, lilting feel with imaginative instrumentation. The band is versatile: two percussionists, bass, Celso Duarte on harp, guitar and violin and arranger Paul Cohen, who switches between sax, keyboards and a spot of fluorescent baseball-juggling for the Mayan-language Come Armadillo.

Downs has an expressive face: frowning, smiling, delivering the words with passionate advocacy. And she can change the sound of her voice with amazing skill, taking on the roles of downtrodden worker, brutal gringo, lovesick suitor and persecuted migrant in turn. Provocative lyrics are married to music that is upbeat and fun: when the shoeless, illegal-immigrant narrator of El Bracero Fracasado is caught, the band strikes up an ironic Star-Spangled Banner.

As a bandleader, Downs is like no one else: strong, independent, feet apart, head tilted back, often singing with a little drum strapped across her chest. Her music celebrates and explores her mixed ancestry - Mexican, American, Indian - and beats a drum for social justice. At La Linea, she gave us a politically pointed medley that rejigged Woody Guthrie's This Land Is Your Land in a pop-Latin style, working in some blunt questions: "When did you come to America the free? Who are your ancestors, what is your creed?"