Manu Chao, Piazza Del Duomo, Milan

For 20 years now, the itinerant Franco-Spanish singer Manu Chao has been peddling a cosmopolitan strain of agit-pop that champions the causes of society's immigrants, invisible and unheard. "I feel a special solidarity with people who don't have papers simply because they were born in the wrong place," he told El Mundo in 1998. "The truth is that I am ashamed of my First World nationality."

In the UK, Chao is still best known for fronting the ethno-punk group Mano Negra. But in mainland Europe, where his new Proxima Estacion Esperanza album is currently No 1 in several territories, he is as popular as Travis are here. Remarkably, Chao's 1998 solo début, Clandestino, sold over three million copies without any kind of promotion, and he still makes great use of what he calls "Radio Bemba", or "word of mouth". Part of his appeal, one senses, lies in his rejection of all that is exploitative and insincere.

The setting for tonight's free gig is majestic, and over 100,000 people have crammed into Milan's cathedral square. Fans gaze down from the crow's nests of lamp-posts, and others have climbed on to statues. Stage right, the Duomo itself is bathed in the magical pink light of dusk. Though Milan has a strong socialist tradition, some are surprised that the city's right-wing mayor, Gabrielle Albertini, has approved this politically charged event. Perhaps he has one eye on the youth vote.

Whatever the import of Chao's message, his reggae-, ska-, Tex-Mex-, dub- and salsa-infused sound is joyous and celebratory. Thus, while "Welcome to Tijuana" draws attention to the plight of Mexican migrants seeking to leave Tijuana for San Diego, it's also a frantic fusion of trombone-bolstered world beats. Tonight it's Chao's 40th birthday, and as the song unfolds, he and his eight-piece band take it in turns to sprint to the lip of the stage. Responding to this, the whole of the Piazza del Duomo seems to throb as one.

Chao is a great campaigner against the economic exploitation that taints globalisation, but his music exemplifies the cultural richness that often results when different nationalities meet on equal terms. His songs may be sung in French, Arabic, Portuguese or Spanish, and his band members hail from France, Venezuela, Senegal and Spain. Hence the overall performance feels like an aural odyssey, and judging by the massive roars that erupt when each medley of songs finishes, everybody is happy to be on board.

One of Chao's own heroes, it seems, is Bob Marley. Further in, "The Marijuana Song" – it's brass riff referencing Mungo Jerry's "In the Summertime" – offers the perfect segue to "Mr Bobby". "Hey, Bobby Marley, sing something good to me," intones Chao, his timbre now paying obvious homage to the Jamaican superstar. To his right, his Senegalese band-mate Bidji raps while whirling like a dervish.

Around midnight, the huge, unpoliced crowd begins to dissipate, while backstage, Chao's birthday celebrations continue with an accordion-led version of "When the Saints...". I feel frustrated that I couldn't understand speeches in Spanish about Mexico's Zapatistas and July's G8 conference in Genoa, but it was still a life-affirming, thought-provoking evening.