The corridors of the InterContinental hotel smell of the uneaten, unchanged and unloved. The mezzanine floor is a miasma of exhausted, reconditioned air and scream-absorbing carpet. It’s one of those constructions of bleak, utilitarian comfort that make you despair of human ingenuity. Here, past the press/business facilities, is a conference room – one of those large open spaces at a premium in cities, used for the amplification of grand, commercial lies and celebrations.

It’s 10am and the room’s been fitted up for a big day. Men in shorts and work boots, wearing T-shirts boasting past crusades and convictions, move about with the concerted, head-down purpose of an imminent, immovable deadline. They are the international freemasonry of fitters, riggers and roadies. Blokes who are never without a corkscrew, a light and four yards of gaffer tape.

Every public event, anywhere on the globe, is built by these leftover medieval artisans who step lively, yelling unintelligible single syllables, testing sound systems, putting up podiums, lighting interactive screens and constructing bulletpoint display boards with an air of smiley cynicism. You just know that at the final trump, an army of these bantering blokes with “Iron Maiden on the road” hair will rise up and dismantle the world overnight.

Keeping out of their way are other men, in suits, with airport ties and multiple phones. They text pleading demands into BlackBerrys. In turn, they are circled by girls with high heels and fussed hair and get-off-me grins. This is the public-relations commissariat. They go with the riggers the way vultures go with jackals, employed by customers who want to avoid relations with the public. To a man and woman they look wrung out and worried like sheep. They are employed to be wrung and worried, to suck up and absorb all the anxiety of the organised, money-go-round world. Today, they’re overseeing the mise en scène for a photo call: a simple stage, a backdrop, a couple of cardboard posters saying something portentous, sentimental and forgettable. It might be a product launch, an employee-of-the-month award, or the declaration of an invasion.

Actually, it’s the stage for Nelson Mandela to have his 90th-birthday picture taken with a hundred folk he doesn’t know, but who know him. More precisely, it’s the stage for a hundred people to have their picture taken with Mandela.

Stephen from Budapest, an economic refugee drone, pushes a vacuum cleaner across a tennis court of nylon carpet. He wears the dull overalls of public invisibility. He’s too old to be chasing dust in a foreign country, prematurely bald with sad eyes. He says he likes London, what he’s seen of it. But he’s tired and, no, he doesn’t know what this event is.

Terry O’Neill, the chosen photographer for Mandela’s birthday tour, is sitting on the far wall while his assistants set up the big-format camera and the synchronised, megawatt hose-you-down lights. They check the focus on a chair for the umpteenth time. We all stare at it. The chair is the centre of the room; the riggers and roadies regard it with a professional respect, as if it were the apotheosis of Meat Loaf. It’s a plush, overstuffed chair, a Louis XXXIII chair, the sort of international chair used for filling up corners in big hotels.

“Go and see how it looks,” Terry tells me. It feels mildly sacrilegious to sit on the chair, like sitting on the throne while the monarch’s having a pee. Already, this ugly piece of hotel furniture is imbued with the saintliness of Mandela, and he hasn’t even sat on it yet. The assistant snaps the shutter at me, pretending to be the best person in the world, and Terry, pretending to be Bono beside me. He’s better at it than I am, and rubs the Polaroid, like trying to get warmth into a corpse.

He looks at the chair and says: “Change it. Change it for something simpler.” The chair’s moment is over. It goes back to being just hotel furniture, a nearly-chair.

This is all displacement activity. The photographers are bored and nervous. There will only be a few seconds for each shot, no margin for error or interpretation or finesse. No space to tell them they’ve got something in their teeth. Around the edges of the room are the napkinned trestles of inhospitable hospitality – the Thermoses of stewed, lukewarm, coffee-style bitterness, the plates of biscuits you’ve never seen in a shop or in a home, that only exist in corporate hotels. Guarding them are girls from the distant heart of Europe, who stand in unflattering uniforms waiting for something big to turn up and rescue them from the biscuits.

More people drift in and collect in the room’s dark marches. Sleek, older men with carefully managed grey in their hair, in better suits, worn with sleeker shoes and silk ties chosen by a tie-choosing assistant. Each of these men has another man or two for their support and affirmation.

These are the captains of consumption, the herders money, the gents who move the First World and shake the Third. Blue-chip CEOs who sponsor and fund and finesse this occasion, this happening. They have come to be photographed with Mandela: a small memento, a religious image to be displayed discreetly on the huge desk or in the annual report, or above the bar in the den. You can tell they’re not used to being kept waiting – they rather enjoy the hair-shirtness of it. It’s sort of appropriate before meeting a saint. In the topsy-turvy world of charity and celebrity, these plutocrats who have chancellors and finance ministers on their speed dials come at the bottom of the cheek-pecking, arm-clasping, back-slapping order. The talent comes first, and they are being corralled in a separate holding room.

One of the PR ulcers whispers “Bring on the celebrity” into his secret-squirrel radio, and through the big double doors they come gambolling like spring bullocks, beaming and dazzled by each other, the occasion, the event, but mostly just by themselves for having got here to this room. What a relief to be excited and giggly, not to be surly and cool, to talk-all-at-once. In one corner, behind a hastily erected cordon for their own safety, are the cast of some multicultural, Milk Tray-coloured, feel-glad-and-horny musical. These perky, pretty teenagers have been mainlining adrenaline and endorphins. They started out as nature’s hyperactive show-offs, but now they’re levitating and vibrating with excitement, tumescent with joy and hormones. They involuntarily bop into little routines, jigging and staggering with a manic, incoherent happiness. They’re hazy about who Nelson Mandela is, but they all know Will Smith is awesome.

Slightly apart is an old black man in a suit, the arms too long. He is unmistakably African. He’s brought the stillness with him; the guarded eyes, the private face, the quiet politeness. He is Mandela’s driver and has been since the beginning – since Robben Island. He looks at the gaggle of humming talent – it’s not the worst thing he’s ever seen.

Behind him is an open door and slowly, like the entry of shadow puppets, a phalanx, a protective creche of people, sidle across the light. In the centre, one shuffling silhouette is unmistakably the figure of Nelson Mandela.

Walking with a stick, supported on each side, the radiant tableau moves towards the arena. The pop stars and the dancers, the plutocrats and PRs and photographers and all their assistant managers, handlers and agents don’t know that he’s just there and Mandela doesn’t yet know that they’re just here. It’s the lull between colliding worlds.

He comes into the room slowly, slowly, his damaged feet in big, comfy slippers, sagging in the trademark African shirt, and they erupt – whistle and clap and whoop, and generally can’t believe it. It’s like The X Factor.

Terry says: “Let’s get the group shot over first” – he doesn’t know how long he can hold back the hyperventilating show bunnies. Mandela is ushered to sit in the middle, in the new star chair, helped by his formidable Afrikaans secretary, one of those terrifyingly admirable women only Africa produces. He looks up and around, like a child who’s just woken in a strange room, and sees all the folk who have come to see him, and smiles this brilliant, beatific smile, a smile that could break your heart. It is the most conscience-tugging, soul-moving facial expression in the world, and he got it in jail. Go back and look at all the photographs of Mandela before the island, and he is another righteous black lawyer with the ebony, private face and the guarded eyes. But he comes out 27 years later with this miraculous face, moulded and creased by injustice into a transcendent African mask, this expression that speaks every language.

The stars fall in behind and around him, like confetti at an arranged marriage. Annie Lennox sits on his left, transported. The seat on his right is kept empty. Will Smith slips in behind him, flicking peace signs. The pack of drama-school kids squirm at his feet like labrador puppies. Where you sit is important, not just because closeness is a blessing but because this picture will be cropped for the papers and social-gossip magazines, and only those at the epicentre are going to get the publicity fix. The empty seat on the right is filled by the ample thighs of Leona Lewis, Simon Cowell’s latest vibrato-rich crooner of the beyond-irony transfusion, Bleeding Love.

“Smile. Look this way, smile,” the snappers call. “Madiba, Madiba, over here, Madiba!” It’s a familiar patronymic of respect, but in a cockney accent it sounds like mockery.

Mandela regards the camera only fleetingly, not ignoring it, just perhaps not entirely aware of it. His eyes wander around the room at all these people, all this attention, all this expectation, all this love. And he smiles the smile. Behind him, Will Smith – who by happy coincidence is promoting his film Hancock – grins and leans forward with the Vs, and everyone else arranges their hands in the semaphore of pop eloquence, and their faces into that contortion of egotism and charity. The flashes flash, the moment is caught, Terry turns away and gives me a look that isn’t in the camera, and I wonder who on earth thought a pop concert was an appropriate gift for a 90-year-old man with bad feet.

But then, pop concerts seem to be our culture’s response to most things – the catch-all celebration, commemoration and commiseration of everything: the Queen’s jubilee, famine, hurricanes, birthdays and small, messy wars.

And why here? Why does Mandela come to London for his birthday? Simply because here he can raise money to combat Aids. We could have sent him a cheque, he could have sat at home, but then these people wouldn’t have had their photograph, and we wouldn’t have had the pop concert.

Now it’s one-to-one time. The entertainers sit next to him looking nervous, some hold his hand, put an arm round his neck. Beside me one of Mandela’s carers mutters, “Don’t touch him,” with sadness and anger. But only I hear it.

Through all this, he sits quiescent and patient, listening without hearing, knowing without understanding, smiling the great warm smile. His fearsome secretary is fearsome on his behalf, moving people on, holding them back: “No managers today, no hangers-on, just performers.”

When the talent’s all done, the businessmen scuttle in, uncomfortable in their suits, fiddling with their tie knots like vestigial scrotums, taking Mandela’s hand firmly and meaningfully, looking deep into the camera’s blinking eye, as if sealing some moral deal.

Why do they want all this so badly? This picture with an old African whose deification by the West is such an indictment of all other Africans. We can’t help adopting worthy people from the developing world – suspending criticism like debt to make them worthier, investing them with a Christmas tree of wholesome goodness and blameless simplicity. There is more than a stain of racism here, an echo of colonial assumption. We see a good man and promote him to an impossibly great man by virtue of his having come from a bad and frightening place.

Poor, dark, benighted Africa; a continent of corruption, violence, megalomania, ignorance, sickness and superstition. How much greater is Mandela in our eyes because he’s risen from the heart of darkness rather than, say, Tewkesbury or Oslo. They hug him and get speechlessly lumpy because he inoculates us against the fears and prejudices about the Third World in general, but Africa in particular. In the process of worshipping him, he is allocated patronising characteristics – an innate, natural wisdom, an avuncular sympathy for children. Someone here said that Mandela was “Gandhi for our generation”. This is a wishful, fairy-tale nonsense that denies him his history, his anger, his blackness, his Africanness. Mandela was a revolutionary. Far from a pacifist, he organised and led Spear of the Nation, the terrorist wing of the ANC. In adopting Mandela as a smiley face, T-shirt slogan, album-cover vision of happy otherworldliness, we take him from his real family: southern Africa. We make him the West’s boy.

He is a real hero, he’s an African hero. His struggle and his triumph are their struggle and triumph. We can’t buy them with an Aids donation and a song. He’s called Madiba because of his black Africanness, not despite it. He should be at home with his family on his birthday. Africans venerate age. He has lived twice as long as most African men can expect to. Here, we respect the old because they were once vital and young. In Africa they respect them for managing to get old.

There is a votive quality about these images – the great and the powerful once had themselves painted with saints to fool God and the neighbours. There’s also something less edifying, something of the game-hunter’s trophy shot.

And it’s over. Mandela is helped to his feet, slowly, slowly. The performers and plutocrats crowd the exit, stabbing BlackBerrys, shouting into their phones for cars and lunch.

Mandela is handed his ivory stick, and hobbles painfully. This, then, is where the long road to freedom ends. He looks exhausted, beaten down by the adoration of strangers, tired by the demands of so much love.

This has not been an edifying occasion. It’s nobody’s fault – everybody had good intentions, or at least enough good intentions. But altogether it’s been sad, dispiriting. Nobody comes out of the photo call looking good. Already the roadies are tearing down the evidence, packing up the metal boxes. The photographers look at their screens, feeding the images into the river of celebrity and curiosity.

As he gets to the door, for the briefest moment, Mandela pauses and looks at the wall where, unnoticed, the coffee girl in her uniform of invisibility still stands guarding the biscuits. She smiles at him. And Mandela bathes her in the great, hot African sun of his grin, and there are tears in her eyes.

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