Moving as one through the record-breaking crowds, half-a-dozen camera crews, ravenous for outrage, swung around to capture the source of the incident - which turned out to be two young Italian artists who had just created a performance piece directly in front of Gilbert & George, entitled Gigolo.
Whether Gigolo was an artistic tribute to Gilbert & George, or some form of protest at either the artists, the gallery, or both, was unclear. But in the momentary hush that had followed Gigolo's blood-curdling howl and thud of hefty boots on varnished parquet, you could hear, quite distinctly, the courteous warmth of George's voice - a virtual parody of Britishness - as he answered one performance with another: "Thank you very much," he said, in the polite tones of a rather grand relation receiving a box of After Eights as a Christmas present, "That was very nice." And Gilbert, smiling, agreed.
In order to understand the cultural significance of the Museo di Capodimonte, and the matching significance of such an institution hosting a major exhibition by Gilbert & George, you have to imagine London's National Gallery housed in Hampton Court and situated in a port city such as Liverpool or Newcastle. For the next few months, The New Testamental Pictures by Gilbert & George, with their titles like punk novellas - Shit On Us, Spunkland, Piss Heads - will be exhibited beside an Italian national collection of Renaissance treasures, in a former royal palace that commands a view of a depressed but defiantly beautiful coastal city.
"But we have always loved Naples very much, since we first exhibited here at Lucio Amelio's gallery in the Seventies," says George; "It's extremely exotic," says Gilbert, with a winning, if enigmatic, grin. "But do you know, this is the first time we have exhibited in a museum and not been asked to withdraw at least one of the pictures. It's because Naples is a port, and they are used to everything here. They won't be shocked. They are open to all kinds of ideas."
Standing side by side at their private view, receiving with unwavering smiles and deferential half-bows a steady stream of slightly hesitant but increasingly devoted well-wishers, Gilbert & George look as though they have just stepped down from one of their pictures. Which, in many ways, they have. For a little more than 30 years, since they first donned their armour of matching suits, they have maintained their public and artistic image in an epic of self-portraiture. Over the past two years, in their "Fundamental" and "Testamental" series of pictures - which have yet to be seen in Britain - they have posed, naked or besuited, against magnified images of their own bodily essences of blood, sperm, sweat, urine and faeces, in the photographed structures of which they claim to see the maps and mystical calligraphy of their own existence and human destiny. They are probably the only artists who have literally put their "everything" into their own work. "People have said that our pictures are difficult to look at," says George. "But they are much, much harder to make."
And because of the uncompromising imagery in their pictures, which has been construed by some critics to offend just about everybody, Gilbert & George have sometimes been described as reactionary monsters. "But when people come looking for the bodies of murdered teenage boys," says George, "we tell them that they may dig anywhere in the garden - providing that they don't dig in that corner over there!"
If the test of modern celebrity is the speed and efficiency with which you enter the mainstream of popular culture, then the sheer scale of Gilbert & George's fame is not to be underestimated. The usually sedate world of the Antiques Roadshow was thrown into momentary disarray when someone popped up with an extremely rare work by George, made prior to his meeting with Gilbert. The television expert designated to assess the all-important insurance value of the piece had to inch his way around not only the precise figure, but also the reasons why this George person has become so important. Explaining the significance of subsequent works by Gilbert & George, with titles such as Blood on Spunk and Shit on Spit, to an audience primed to appreciate the curves of a Chippendale commode, proved testing. Similarly, Gilbert & George have been the subject of a lengthy sketch by French and Saunders, while their performance on The Last Resort, dancing to "Bend It Shake It", is one of the most requested repeats from the show. More recently, Virgin Records' new year sale has been advertised with window- display posters based on both the format and the magnified images of blood used by Gilbert & George in their "Fundamentals" pictures; even the slogan "Bloody Big Sale" is based directly - right down to the typography - on Gilbert & George's iconic title, Bloody Life. Gilbert & George, as a cultural concept, uphold the definition that Brian Eno once gave of pop music: "It's about creating imaginary worlds, and inviting people to join them."
At precisely 8pm, with Italian functionaries' uncompromising need to close on time, the gallery attendants began to usher the crowds down the broad flights of imperial stairs which led to the darkness of the surrounding park. And after a slow procession - suitably regal, but always ready to pause for an informal snapshot, Gilbert & George left the building. "The thing about Gilbert & George," said one of their few close acquaintances, "which is almost as important as their art - and their art is very important indeed - is that they have survived as a couple. They are even preparing for their deaths, I think, and that is a proof of some immense depth to what they are to one another."
Later, at a dinner held in their honour at a restaurant on the waterfront overlooking the Bay of Naples, Gilbert & George sat side by side, as always, dispensing a warmth and cheerfulness that seemed to inspire the entire restaurant of bemused Italian families and distracted lovers. Around midnight, they rose to sing "Happy Birthday" for one of their guests, conducting the crowd with their upheld glasses of wine. In some magical way, the dinner itself had become a performance - a happening. Finally, a rumour circulated that an exhibition by Gilbert & George might be the opening event at a major new gallery in Milton Keynes. This, given their ability to outrage both the public and the art world, would be bound to turn a few heads.
"We found out that there is only one piece of graffiti in Milton Keynes," said George; "and it simply says `Vicar Says Yes'." As ever, Gilbert & George are giving nothing, and everything, away.
`Gilbert & George: New Testamental Pictures', Museo di Capodimonte, Naples. To 7 February