Three years into its existence, Africa Express still looks like something no sane person would attempt, a triumph of idealism over the practical difficulties involved in pulling together a mammoth show that's part marathon gig, part mini-festival, most of it done on the fly. Throw in the hurdles that come with attempting to unite diverse Anglo-American and African artists in a series of improvised collaborations, and the potential for disaster is obvious.

Then again, you don't come up with an innovative, often surprising method of putting both sets of musicians on an equal footing by being risk averse. It works, too, with Africa Express doing as much as anyone to consign that always uncomfortable, loaded phrase "world music" to a less enlightened past.

Today's five-hour show in Paris, a highlight of a free local festival held in front of the town hall, is something of a victory lap, the biggest Africa Express event so far, with an estimated 15,000 people turning up to see more than 100 musicians. Grander, then, than their ventures at Brixton Academy and Glastonbury in 2007, as well as the nine-hour marathon in Liverpool and shambolic appearance at the Electric Proms the following year (though perhaps not as adventurous as stop-offs in Mali, Nigeria and the Congo). The British contingent even pitches up the same day, straight off the Eurostar, like a school trip entirely made up of the interesting kids from behind the bike sheds. Among the pupils: Super Furry Animals' Gruff Rhys, Corinne Bailey Rae, making her first live appearance since the sudden death of her husband in March 2008, and Damon Albarn, one of the project's prime movers.

While French posters advertising "Africa Express avec Damon Albarn", aren't quite in the spirit of the exercise, they do acknowledge his role as ring leader, a catalyst who spends much time enthusiastically marshalling potential combinations "backstage" at the town hall, in the huge civic function room, festooned with chandeliers, that's doubling as rehearsal space. Outside, the set itself takes a while to bed in, with the opening gambit - Malian singer and guitarist Vieux Farka Touré joined by Bailey Rae on backing vocals - being more gentle introduction than dramatic overture.

A necessary jolt of energy comes from Chicago's raucous Hypnotic Brass Ensemble - think big-band brass played with a hip-hop swagger - but it's not until a regal stint from Malian vocalist Oumou Sangare that the crowd - a cross section of picnicking locals, passing tourists and a decent west African contingent - come fully on board. Sangare's easy fusion with the Kick Horns, who add firepower without treading on anyone's toes, suggests subtlety can occasionally trump the search for explosive connections.

Inevitably some experiments fall flat, such as the Kooks' hammy cover of the Cure's "The Lovecats", which shunts Moroccan singer Hindi Zahra aside, but others are smile-on-the-face silly, like Gruff Rhys's romp through solo track "Gyrru Gyrru Gyrru", assisted by Albarn and local star Catherine Ringer. Plus the last 90 minutes are a terrific blast of controlled chaos, with the organisers still trying to give everyone a go; not easy when there are enough musicians to play on until sunrise.

So while a burst of fearsome rattling and droning from the Congo's Konono No 1 leaves you wanting more, a swift negotiation with the chief of police adds a 15-minute extension to the midnight curfew, leaving room for two finales. The first is an all-star jam anchored around veteran Nigerian drummer Tony Allen, the second culminates in French-Algerian Rachid Taha leading the entire cast through his blood-and-thunder reclamation of the Clash's "Rock the Casbah", with mischief in his eyes and a top hat on his rascally head. That's a part-European, part-north African leading an international line-up through an Arabic version of a British rock song. The Parisian audience? They're dancing.