Michael Glover joins an Ascension Day procession in Bruges

Religion and Bruges are as inseparable as the three persons of the Trinity – or, in the jocular words of our guide: "The blood of Jesus Christ makes tourism popular in Bruges." Its great Gothic churches dominate the skyline of the medieval city; representations of saints and the madonna fill niches in walls on many street corners; its museums are full of religious paintings by Van Eyck and Hans Memling, among others. And in May, on Ascension Day, the Procession of the Holy Blood takes place, an extravagant display of civic and religious pomp held here almost continuously since the Middle Ages.

This year, half an hour before the parade begins, the atmosphere in the raked stands which overlook the old market square with its soaring medieval belfry is like that of a football crowd – excited, almost raucous. One ingredient alone is missing: violence. The crowd pressing against the crush barriers – there are 50,000 people here – are bourgeois to a man and well-coiffed woman. A young priest in his black soutane, with matching black anorak is leading a boy and girl by the hand. But why are we all here?

Legend – or perhaps history – has it that in the aftermath of the Second Crusade, Derrick of Alsace brought back a holy relic to the city: a specimen of the blood of Jesus Christ, which had already been miraculously preserved for more than a millennium. It is kept in the Basilica of the Holy Blood, where, according to a notice in the stairway en route to the Upper Chapel, it can be venerated on Fridays. On Ascension Day the relic, sealed within its rock-crystal flask, and contained inside a magnificent, 17th-century gold and silver shrine by Jan Crabbe, is venerated.

Some 1,800 people participate in the long procession, which takes an hour and 20 minutes to pass us. Part pageant, part a string of narrative episodes from a collection of medieval mystery plays, it re-enacts the principal events of the Old and New Testaments, and then concludes with a solemn parade of civic and religious dignitaries, with the shrine itself bringing up the rear.

Bands, some 60-strong, blare triumphally; horses, camels, sheep, hawks and rabbits have their parts to play: God, in a grave Old-Testament wig, runs hither and thither, fulminating against the unbelievers; Jesus dramatically falls to the ground beside his unbearably heavy cross. St Veronica holds up to the crowd the handkerchief with which she has wiped his brow. And, lo, it is imprinted with the image of his suffering face! It all ends with a blessing from the Bishop, who raises the relic in our direction, up, down, up, down. "Countless graces," intones the official handbook, have accrued to the city, "... through devotion to the relic."

Our appetites whetted for all things religious, we make our way back to the Gothic church which stands beside the old Town Hall on the Burg, the city's central square. The Basilica consists of two chapels, an upper and a lower. The upper dates from the 19th century – the original chapel was destroyed during the French revolution by groups of men collectively described by our guide, with some venom, as "Bruges Jacobins".

The Basilica's Upper Chapel is decorated with a suite of narrative paintings derived from apocryphal stories also used in Jan Van Eyck's great altarpiece, The Adoration of the Mystic Lamb, in Ghent Cathedral. The stained glass windows are 19th-century, too – the originals are in the Victoria and Albert Museum in London. The most important room on this floor is the adjacent Treasury, which houses the shrine of the Holy Blood itself, designs for those stained-glass windows, and many other curiosities.

The more interesting of the two chapels is the recently restored lower one, which is as austere and dark and Romanesque as the upper is cluttered and bright and Gothic revival. The pelican on the high altar represents Christ himself, because it is said to pierce its own breast for the blood with which to feed its young. The huge circular columns in the chapel are unusually various in colour and appearance; one is made from blue Tournai stone and another from a sandier, volcanic rock.

Other churches worth visiting are the Church of our Lady, which contains a white marble "Madonna and Child" by Michelangelo – one of only a handful outside Italy – and the Gothic Cathedral of Saint Saviour, which is said to be the oldest brick building in Belgium.

But for the great Flemish religious paintings – altar pieces, portraits of suffering saints and much else – you need to visit the Groeninge Museum, and the re-opened St John's Hospital/Memling Museum, which houses a handful of Hans Memling's finest works.

The Facts

Getting there

The author travelled as a guest of Eurostar (08705 186 186; www.eurostar.com), which offers return fares to Bruges from London Waterloo from £79 return. The journey time is around three hours, 40 minutes. Double rooms at the centrally located four-star Hotel Heritage (00 32 050 444 444; www.hotel-heritage.com) start from €130 (£93).

Further information

Contact Visit Flanders (0906 -302 0245, calls cost 60p per minute; www.visitflanders.co.uk).