Travel agenda: The original trance-dance

After a long, dusty drive from concrete Agadir, Essaouira appears in the distance like a shimmering citadel. Hippies, tourists, surfers, refugees and loners have long found sanctuary in this southern Moroccan town, far from the hassle of the northern cities. Even before its heyday as a trading port in the 18th century, sub-Saharan Africans shipped up here, seeking the produce of Europe in exchange for gold, salt, ostrich feathers - and slaves, who brought their own and only precious commodity, Gnaoua.

Like the blues, the music of Gnaoua evolved from rhythms beat out to ease the suffering of the soul. It is still chiefly concerned with the healing of spiritual wounds, using trance, prayer, possession, exorcism and dervish-like dancing, with music made from instruments unique to the tradition: the guenbri, a long-necked three cord lute, the qraqeb, metal castanets, and ganga drums.

Gnaoua took root in this area of Morocco, and its rituals call on pagan deities or mlouk, as well as honouring Islamic saints, claiming spiritual descent from Sidna Bilal, Mohammed's son-in-law, who became the Prophet's first muezzin.

With the rise of World Music, Gnaoua is fast gaining recognition outside Africa, and this year Essaouira is hosting the first ever international festival of Gnaoua, billed as the original trance-dance music.

The three-day festival will take place on 5, 6 and 7 June, with six concerts planned. Five of the most famous Gnaoua groups will be in action, including Amida Boussou, Abdeslam Allikane, and Mahmoud Guinea (who played with Santana in Casablanca), Bakboh from Marrakesh and Hassan Hakmoun, signed to Reelworld. Also playing will be Berber musicians called Gauga, allied to Gnaoua. These musicians are not only masters of their art, but seen as givers of brotherhood and doctors of body and soul. The festival will also include a conference on the culture of Gnaoua, and the highlight will be the sacred night of the lila or derdeba.

The lila is a night of ritual possession and divination. Amid prayer and incantation, ritual dance and handclapping, 40 or so spirits and ancestors are invoked, roughly corresponding to categories of human sensibilities. Next come the fumigations, the sprinkling with rose and orange blossom water and the musicians play the guenbri, always sensitive to the audience and reactions of those in trance.

Yet this is not just a matter of a medieval Christian-style exorcism, expelling evil spirits from those placed in a trance, but also of inviting possession: the genies dance in their bodies as a healing power, to the music played by the Gnaoua musicians. At dawn, breakfast is taken and the company returns to the secular world.

The trances are quite something to behold, from trembling to raving to epileptic-type fits. But clairvoyants and healers are on hand to avert accidents. Unlike Arab society at large, women are not excluded or secondary but central to the proceedings; the master of the santuary can be a woman - as are many of the spirits invoked, and the clairvoyants are usually women, chosen either by hereditary gift or a revelatory illness; sickness is seen as a sign you have been chosen by a spirit. The devotees and the trancees are often predominantly female.

Stories abound of Lazarus-like healings, of physical and mental disturbance cured. A Dutch professor who had been paralysed for 30 years was found to have nine devils in residence. The healer brought out the oldest devil, reading from the Koran, the other devils having died long ago, and the professor walked.

Jane Loveless, the festival's organiser, witnessed a healer heating metal in a saucepan, which was then held over the head of an 80-year-old woman who had suffered a paralysing stroke. Water was poured over the metal, the healer then read the forms of the newly solidified metal to the sound of incantations. The woman began to recover immediately and within three weeks had regained total use of her limbs and senses.

All events at the Gnaoua festival are free, though the official lila is by invitation only. However, there's every chance of spontaneous lilas breaking out all over town in response to demand. Celebrations have a way of spilling out over the streets, and many events are programmed around town to coincide with the festival, with music, exhibitions and films pertaining to Gnaoua.

Essaouira is a small town that has prospered greatly in the last 10 years and has become a welcoming haven for all, with an easy rhythm of life, European and Berber cafes, fantastic spice markets, good hotels at all prices and various illicit drinking holes. Around the harbour fishermen shake flailing octopus and fresh fish to tempt passers-by, before grilling them for all to eat at trestle tables; behind them the great orange hulls of ships-to-be lie stranded on the port, and beyond that the beach extends as far as the eye can see.

The best airport with regular flights from Britain is Marrakesh, served by British Airways (0345 222111) from Gatwick and Royal Air Maroc (0171- 439 4361) from Heathrow via Casablanca. For travel out on 4 June and back on 11 June, Hamilton Travel (0171-344 3344) has a fare of pounds 259 on RAM. From Marrakesh, it's about four hours by bus to Essaouira, for pounds 3. For details of the festival contact Jane Loveless on 00 212 4 47 63 47.