Preview: Proms On Four - World Music Celebration, BBC4

There's magic in the music of Morocco

Guitarist and producer Justin Adams is walking along the main shopping street in Essaouira on the first evening of Festival Gnawa, a free annual event that transforms this beautiful port into Morocco's biggest musical gathering, with half a million people attending last year's 10th anniversary celebrations.

He's here with Faluni master musician Juldeh Camara and English percussionist Dawson Miller to perform music from their album Soul Science, winner of the best crossover album at this year's BBC World Music Awards. They've gigged everywhere from Mexico to Glastonbury to the award-winners' Proms concert – but an invite to Festival Gnawa is special.

Gnawa has ancient roots – close to the blues, but closer to the world of spirits, magic and ritual. Outside Morocco, it has a cultish following, but here it's huge. "You had to search it out," Adams says. "You'd get glimpses of it in Marrakesh, but you didn't feel you were seeing the good stuff."

Adams's taste for gnawa runs to the pure form. "I've got these wonderful old pictures of gnawa musicians playing in the 1950s. This was like seeing one of those pictures come to life on the street," he says. "What was fascinating was how the audience sang along. It's fantastic that a traditional form of music is seen by young people as very cool."

Moroccans stream in for the festival, camping on the beach where the club stage pumps out dance music and gnawa fusion. Europeans are discovering the festival, too. Essaouira has a history of close contact with Europe, especially England, its main trading partner when it was called Mogador and was Morocco's major port. It was an Englishwoman, Jane Loveless, who started Festival Gnawa in 1998. Now it attracts major artists from around the world, recently including Wayne Shorter and Toumani Diabaté.

'Proms on Four: World Music Celebration' with Justin Adams, today, 7.30pm, BBC4