On the desert fringes of Omdurman, in the middle of a vast, sandy burial field, the dervishes begin to spin. For a hotline to Allah, look no further
"Ohh," bubbled Usman, "oh yes, this is something interesting, very interesting." He doubled up and his eyes became mock fiery. I backed off, leaving room for his mime. Slowly, he pumped his arms in unison, fists clenched, like the pneumatic joy of a footballer. Rocking on his heels, folding his long body from the hip, Usman was beginning to frighten me. Suddenly his impression broke up in hysteria: "Ohh yes, Mr Hugh," he said, "the dervish, very interesting to the foreigner."

In Sudan, the Sufis are regarded as eccentric, even by their own countrymen. Calling someone a dervish is an endearing way of saying he's crazy, and is accompanied by the ubiquitous swivelling finger on temple. But a dervish is only one, though undoubtedly the most spectacular, type of Sufi in Sudan.There is a plethora of "tariqas" or sects in Khartoum, each with a subtly different garb and dogma. On Fridays, as most people head through the sandy alleys, called to the mosques for prayers, the Sufis, often in green gowns, natty and excited, gather on street corners in ragged and accumulating groups: two ways of worship.

But dogma never creates tension. All Sufis have one aim: to come into closer contact with Allah. Given that many have committed the whole of the Koran to memory, coming closer to their divinity needs radical energy. Hence the famous whirl.

Usman takes me to the bus-stop. "Hamed an-Niel," we cry and our fare is waved away by the driver. No one should pay for their prayers. Today, 11 July, is the birthday of the 19th-century sheikh whose tomb is the site of Omdurman's whirling dervishes and the Bedford bus will burst if the driver takes another pothole at speed. Our bus swells with the infectious and jerky music that I came to associate with converted cattle trailers and a heaven-sent breeze. Four men perch on the back step, clutching the swinging door, pinching their faces against the wheeling dust. Four white jalabiyas flap hectically in the wind.

On the outskirts of Omdurman, the desert is winning the battle that it is still waging in the city centre. Buildings have been started and abandoned and sand rides seductively up free-standing concrete supports. Further out and just visible on the horizon are the refugee settlements that besiege the city. Between us and them lies a vast burial field. Slight humps extend as far as the eye can see, casting shadows that act as gradually lengthening gravestones.

Hamed an-Niel's tomb is the centrepiece of the cemetery. It is square and robust, as big as a chapel and topped by a conical dome. Even from afar, a sense of drama widens from this curious centre out over the flats, drawing people into this magnetic field and arranging them like iron filings around the shuffle of a tambourine. This is the call to prayer: no wailing broadcast from the minaret, but a tambourine man singing the Koran, circling tea-drinkers, fixing individuals in his stare, and establishing an impossibly funky rhythm for the afternoon.

A small circle forms around him while a young man traces arabesques in the air with his hands and lowers mock-timid eyes to the ground. As hearty as a student in drag, he mimics a new bride dancing for her husband. The circle is pleased to see we understand the joke and the young man shakes his hips provocatively.

The light interlude is soon interrupted by a fiercer drum that approaches from the cemetery. The crowd turns to face a sombre column of dervishes swaying towards us under green banners emblazoned with Koranic script. At their head is a terrifying man, dreadlocked and obese and bearing a whip. He charges into the crowd threatening them into a perfect circle with grimaces, pointing his whip gruffly at children and rearranging the front row so that women cannot distract the dervishes from their communion. He is a showman, a ringmaster, the melodramatic director.

Turning to a line of men in the front row, he whips up a long pulsing chant: "Allaaaaaah Allah. Allaaaaaah Allah." The whole crowd now joins in and the chant is soon a slow, awesome moan.

Two or three dervishes have taken their positions in the arena, wearing tunics of different colours and holding rods before their faces. Turning slowly on bare heels, the dervishes test the ground, occasionally greeting friends in the crowd who come forward to kiss them. A cacophony of distorted instruments strikes up and manages to syncopate with the crowd's chant. The whole front row begins to pump arm and torso just as Usman had demonstrated and the dervishes pick up speed, gowns blossoming into wide bells.

At first one can't help wondering why they don't fall over. As a child, I could whirl for no longer than a minute before collapsing with the world heaving round my ears, but a dervish will spin for at least half an hour before being replaced by another. A grin slides over his face and the eyes recede back and up. If successful, the meditation expels all thoughts from the dervish's mind, allowing Allah to fill his consciousness.

As I looked round me, it was obvious that the dervishes were more than just the eccentric side of Sudan's spiritual life. Widespread poverty, illness and a regime that fear has kept in power has brought the Sudanese quality of life to an all-time low. Dervish "monks" not only choose a life deprived of the meanest luxury, but claim to derive strength and energy from it.

The dance is, perhaps, subversive. Drawn from the government-controlled mosques in the city, the people can express themselves in the privacy of wasteland. The dance allows the Sudanese to remain faithful to their religion without having to kowtow to the regime.

At sunset, the crowd disperses through the cemetery and back to the Bedford buses. Storm clouds have gathered and look ominously like they might unleash one of Khartoum's legendary sandstorms. Everyone says how much they are looking forward to the staple bean dish "fuul" and to seeing us same place, same time next Friday.

It starts to rain and the roads are soon mud. As we stare at the passing market stalls and shacks, I wonder where the intensity has gone. The anger and wildness that the dance inspired are gone and the clouds pass, leaving a cooler Khartoum. The week looms and minds return slowly to tomorrow.