There was a moment in the old medina in Fez, not far from the great gate known as the Bab Bou Jeloud, when three worlds seemed to collide - God, commerce and the Wolseley, in Piccadilly.

Here, within a few steps of one another, you find the religious, the secular and an excellent latte with lemon tart. Mammon is everywhere in these lanes, but God and the lemon tart would be all too easy to miss.

Fez is an extraordinary survivor. A medieval Muslim city, little changed in 1,000 years, it offers a vision of a world when the clash of civilisations involved Barbary pirates and white-slave traders.

Still enclosed within high walls, still threaded by a labyrinth of narrow alleys where mules jostle with robed figures, the medina is a whirlwind of buying and selling. If the intensity takes your breath away, it is not because there is more commerce than on Kensington High Street, but because in Fez everything is so brazenly public.

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In the Talaa Kabira, a high street so narrow that two oncoming mules are a traffic jam, it is almost impossible to stand still. The lane is a river of people - here a cascade of schoolgirls, there an eddy of women in black robes, round a corner a calm pool of bearded hajjis, in a side alley a sudden current of men in loincloths hurrying to the tanneries.

Mysterious figures in burnouses with pointed hoods perch on the ledge of a tiled fountain like extras from The Lord of the Rings. Veiled women press round the underwear stalls, checking out the frilly knickers. Families of pale Berbers from the Atlas, blue tattoos wrinkling on their cheeks, crowd into the jewellers’ shops to finger the gold chains. Dark Africans from the other side of the Sahara cast knowledgeable eyes over piles of dried fruit - figs, raisins and dates - each, like the men themselves, the product of a different region. The crowds part only for the muleteers and their barging mules, when everyone flattens against the walls to let them pass downstream. They are the only traffic.

Aromas waft through these lanes defining invisible boundaries - fresh coffee, cloves, olive oil, rose water, urine, mint, the sweet scent of cedar shavings, the stench of tanning leather, the tang of onions frying, the acrid smoke of burning charcoal. Sounds waft, too - childish choruses of recitation from behind the shutters of tiny Koranic schools, the rhythmic clang of coppersmiths, the klaxon shouts of porters, bent double under piles of sheepskins or tottering towers of shoe boxes, the mantric cry of beggars, the bells of water vendors, the muezzins calling the faithful to prayer, never quite in unison, from every minaret in the city as the sun slides off the rooftops.

There is one moment when commerce falters - Friday midday, when the entire city pauses for prayers. Suddenly, the rivers shrink as people make their way towards the great Karaouiyne mosque, to which all lanes in Fez eventually lead.

The Karaouiyne is said to have been founded in 857, long before the earliest of England’s Norman cathedrals. It is one of the great mosques of Islam. In these lanes, though, you could pass the Karaouiyne a dozen times without knowing it was there. In spite of its vast size - there is room for 20,000 worshippers - its blank exterior walls are almost indistinguishable from those of the surrounding shops and houses with which it is so intricately enmeshed. It is only a door, left ajar in a humble lane, that allows you suddenly to glimpse the vast pillared courtyard, the great stretch of open sun, the worshippers assembling for prayers.

On Fridays, all Fez seems to gather in this courtyard. NonMuslims are not admitted, but through one of the 16 gateways you can see the long rows of men sitting crosslegged between the pillars. There are women, too, but in separate porticoes. Once prayers are over, the great assembly rises and makes for the doors, where commerce awaits them.

In the alleys just outside, sock salesmen have set up shop.

I did not find God at the Karaouiyne, but I did discover the divine back where I had begun, at the other end of the Talaa Kabira, near the Bab Bou Jeloud. Fez was the cultural centre of Morocco for 1,000 years or more, and as such it was home to numerous medersas, or Islamic universities. One of the more exquisite is the Bou Inania. Distracted by the chaos of the street, you could miss this miracle of medieval Islam. But you need to notice it. This is one of the reasons to come to Fez.

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