Welcome to the pleasure dome

So there it was at last: an entry ticket to Xanadu. A sneak preview inside the most controversial, most gawped-at and talked-about, mythologised and derided engineering phenomenon in Europe. An invitation to inspect "the Dome so far" from Simon Jenkins, journalist, visionary and millennial commissar, whose legendary fondness for throwing parties in spectacular public arenas has brought him, and 200-odd heavy friends, to this epic patch of east London wasteland, where he hopes to convince sceptical "opinion- formers" of the brilliance of his brainchild.

Who could resist? I felt like Stout Cortez, on getting his invitation to inspect the Pacific Ocean. Like Marco Polo, shortly after hearing he could have an hour's audience at the Court of the Chinese Emperor. It's only a bit of stiff cardboard, I thought (and I expect Marco and Stout thought the same), but it's going to show me a whole continent.

I've always been a fan of the Dome. When leader writers inveighed against it as a senseless waste of pounds 758m, when the tabloids bitched about it as an elitist folly, the architectural correspondents mocked it as a boring, middlebrow tent, and every black-cab driver from Bermondsey to Shepherd's Bush was required by law to say "You could build ten 'ospitals with that kind of money", I nursed a sneaking regard for the thing.

I'd been there, after all, at the birth. I was around when it was just a dozen yellow legs splayed into the air like a spiky crown. Every day, on the way to the Canary Wharf car park, I'd see them poking into the sky before me.

Then the legs acquired a spider's web of silver mesh that hung suspended between them, becoming more densely textured each day. The web glittered in the sunlight; it sparkled gamely in the winter frost. Then, one morning, a single, white wedge of tarpaulin was pulled over one corner. More and more glass-fibre wedges had been stretched and yanked into place each time you looked. Suddenly it was all there - the great white belly, the yellow antennae, each with its thrilling red light shining on the tip at night, the huge hole in its side like a steel ulcer. The Stately Pleasure Dome, at last.

And now I was going to see inside. It would be the Grand Canyon. The rings of Saturn. Niagara Falls. The mother-ship at the end of Close Encounters. The Sistine Chapel...

The first thing to know is, don't go there by car. Wait for the Tube line to open. I got stuck in an almighty traffic jam en route to the Blackwall Tunnel, as advertised on every radio traffic report you've ever heard. "The Millennium Experience Company, Gates 1 and 2" said a yellow sign, indicating left. But I needed Gate 3, so I drove on. Ten minutes later, I found myself outside Charlton Athletic football ground, heading for the Thames Barrier. I turned round, negotiated a dual carriageway, two roundabouts and an unexpected motorway slip-road, and finally, along the charmingly named Bugsby's Way, found it.

Close up, the Dome is brilliant. It's very white, but it's full of glassy sheen and wiry glimmer. The Teflon-coated glass fibre is tautly stretched like canvas, the metal trim is polished, the whole looming structure feels nautical, shipshape, temporarily moored. What from a distance looks more like a floating, horned bomb from the First World War seems, close up, like the world's biggest ocean-going yacht.

Inside, smiling Millennium Experience girls greet the guests with the glassy-eyed delight you otherwise find only in the Disney Shop. You take your glass of chardonnay. Claire Tomalin, the biographer and critic, says hello. She is just back from giving a talk to the Jane Austen Society of the Canadian Rockies, who turned out in mob caps and crinolines to hear her.

Paul Johnson sweeps past, apparently heading for the exit. The place is full of right-wing intellectuals and former Tory cabinet ministers: Douglas Hurd, Michael Howard, William Waldegrave. But here too is Ed Victor, the American super-agent, so it is a real party after all.

Only after five minutes talking Jane Austen with Ms Tomalin, and profiteroles with Caroline Waldegrave, does it occur to you to look at the interior. (It's the eighth wonder of the world. The least you can do is look at it.) On a preview platform, you gaze across its expanse for the first time, every descriptive nerve in your body jangling, and...

You're looking at a big tent. A Big Top with concentric circus rings in the middle. The ceiling, across which 40 "aerialists" will fly during the Peter Gabriel-directed Millennium Show, is a wondrous bomb-burst of a thing, but you get no sense of its scale, only its symmetry. The far horizon could be a mile away, but you feel you could stroll over there in five minutes.

The Dome isn't empty any more. It's full of partitions and machinery and orange seats, and filigreed metal threads hanging down, attached to the circular track that will be yanked up to the ceiling. Gantries and walkways snake and serpentine about. The contentious human sculptures, the joined-together androgynes, are at a rudimentary stage, all wires and blobby outline, as indistinct as Henry Moore jelly babies.

There's a lot of clanging and steaming in this manufactured cavern. It's a James Bond villain's retreat, with a Tom Waits soundtrack.

Everyone who came in said "Bit smaller than I expected", as though advancing a bold theory. "Teensy bit disappointing". They recited bogus believe- it-or-don't statistics to each other. "Apparently you could fit Nelson's Column quite easily under the roof..." "Wembley Stadium, twice over..." "You could get the Eiffel Tower in here, you know. Lying down, of course, on its side..." "I believe you can park 438 buses in this space," the director Kevin Billington told a female design editrix.

"Somebody told me you could get most of Mayfair on to the Dome site," she replied. "Not just in here, obviously, but if you include all the service areas." "Our social lives must be very different," said Billington sadly. "You measure space in Mayfair houses and I in London Transport buses..."

Robert McCrum, The Observer's literary editor and a quondam linguistic historian (The Story of English), surveyed the prospect with professional interest. He's been signed up to supply new names for the innovative features in the Dome's construction. These state-of-the-art stanchions and hawsers and architraves have no English names. What's that fat, curling pipe thing at the far end, Robert?

"Some kind of vent, I think," said McCrum. Vent? Vent? What kind of word is that? I expected something more noble, more cutting-edge: "It's a Shantigrammicon." "It's a Nebulova." McCrum shakes his head. "They turned down my first list of words," he said. "`Too flip and ironic', apparently."

Carping voices were raised, as always happens when British people are shown something and invited to admire it. "You know the huge hole in the side of the roof?" someone asked. "It's a ventilation shaft for the Blackwall Tunnel. Without it, all the motorists down below would suffocate. The Dome people didn't realise when they drew up the plans. Honestly. A conveyancing lawyer doing a mortgage search could have worked that out."

"Of course, it's mimicking the Festival of Britain," said the chap from the Telegraph. "It's all so ridiculously retro. The sponsors are Walls and Typhoo, and Mars and Kodak. This whole thing is England in the Fifties. They had to have a dome because of the Dome of Discovery in 1951." So why, I asked, aren't they building a Skylon this time round? "Oh but they are," he said. "Twelve of them" - and he pointed at the dozen yellow masts that poked through the glass fibre canvas into the night sky.

"This is not an exhibition," said Jenny Page, the no-nonsense chief executive of the New Millennium Experience Company, "at least, not in the sense of being object-based." But the 201 "millennium objects," I insisted - the aqualung, the waterless urinal, the Viagra, the Teletubbies, the Ford Focus, the ultra-thin condom, the landmine clearance system... "They're all designated by the Design Council, not by us," she said. "We regard the products as part of a larger celebration of productivity and innovation." But if it wasn't an exhibition, what was it? "An experience," said this unlikely New-Ager firmly. "The Dome Experience is directed at individuals and ordinary lives. It's not a trade show. It says that the world is a place where marvellous things happen..."

By now the guests, finding themselves inside a large marquee, had started to behave as if they were at a wedding. "I'm sorry for Kathryn Flett," observed the wife of a broadsheet editor, "but I don't think I can read another word about her ghastly marriage." Marigold Johnson discussed the flavoursome quality (and yet the exorbitant price) of organic carrots. I became embroiled in a hot argument about the merits of The Phantom Menace, with someone who hadn't seen it either.

Two consultant engineers, who seemed not to have been involved in the Dome's construction, gazed above them. "Look at those flanges," they breathed. "They're ground-breaking." Robert McCrum gazed at an enormous structure of weights and pulleys, its purpose wholly inscrutable. His lips moved. "Spronglegrummit," I thought I heard him say. "Herpsigraph..."

We had more chardonnay. We tried to ignore the spectacular glass fibre cathedral over our heads, the months of manpower, the thousand bright ideas now approaching fruition, the million feats of inventiveness and daring. Here was pounds 700m-worth spent on a celebration of present, past and future. Here was a historical monument whose like we won't see again in our lifetimes. And here was the cream of London society, struggling to take it seriously.