Artzone

God, the size of it. Whatever Tate Modern may be, it's more than just a gallery. It is an event. No other recent arts project has enjoyed such a triumphant campaign of pre-publicity. No other will have had its opening ceremony televised live on BBC 1 in the middle of the morning, as if it were a great occasion of state - to be followed by more live coverage of its launch party later that evening. This happens on Thursday. On Friday it opens to the public, free of charge.

And what you find inside the giant converted power station on London's Bankside is a contemporary pleasure complex, a total shopping-scoffing-strolling environment, but one whose particular attraction is not rides, nor wildlife nor ancient history - but modern art.

There was a time when modern art was nobody's idea of fun. The lowbrows thought it was boring. The highbrows thought it was serious. But that is changing.

This is an experience, a Modern Art Experience, which I must say I found extremely impressive and exciting. I've never seen anything like it. The curating is original and unflaggingly inventive, with a ready sense of theatre. That is clear straight off. Entering the shockingly large and empty Turbine Hall, you may notice that no art whatsoever is to be seen.

No art - except for one group of works, a specially commissioned installation by the aged French-American sculptor Louise Bourgeois: an enormous spider and three house-high metal constructions, soaring Rapunzel towers with gracefully, perilously spiralling fire-escape ladders snaking around them, and a platform at the top bristling with huge, concave shaving mirrors, which ring the ascended viewer in distorted reflections. The commission could hardly have come out better. The spider is silly, but the towers are solidly visionary pieces, with a touch of fairground, too - stupendous in scale, and frankly like helter-skelters. Exactly the right note.

Elsewhere on ground level there's the shop - the largest gallery shop in Europe (sublime statistic!), with a press release all of its own (headlined "Tate defines the modern art of shopping") and with a stock ranging from texts of radical critical theory to artist-designed fridge-magnets, a feast of sociological study. And for feasts proper, there are on-site eateries lodged around the gallery's seven tiers.

But to the exhibitions.

There are three tiers of exhibition spaces - 85 separate galleries - arranged in webs of communicating rooms. There are punctuating lounges, where visitors can sit, leaf through the literature provided, gaze out over the river, or down into the Turbine Hall. There are videos showing in rooms as big as small cinemas. And there are guiding words everywhere. Has a gallery ever had so many wall-texts and labels? Each work has its descriptive caption. Many have personal appreciations, invited from professional and amateur art lovers. I have contributed one myself. And art? Level 4 is for special exhibitions, and for the launch it holds an anthology of contemporary art spectaculars. There's Cornelia Parker's exploding shed, its fragments suspended mid-air on threads; Rachel Whiteread's Ghost, her famous cast of the inside of a room; Bill Viola's huge, symbolist birth-life-death video piece, The Nantes Tryptich; a labyrinth by Ilya and Emilia Kabakov; a memorial by Christian Boltanski... The display is a good idea, which eventually becomes a bit of a pile-up.

But showing the Tate's permanent stock of modern art was always the gallery's raison d'être. London's collections have been re-shuffled in the process. The Tate has split its collection in two - historic British in the old Millbank building, international modern to Bankside, with modern British torn between the two. And with 1900 taken as the dividing line, the Tate's pre-1900 foreign art has gone to the National Gallery, the National's post-1900 art to the Tate.

As for the display, though - and this is the great talking point - it is determinedly non-historical. This is a modern art gallery that doesn't show art according to the established sequence of art movements - Post-Impressionism, Fauvism, Cubism, Expressionism, Abstraction, Surrealism, and so forth. From 1900 to now, it's mixed up together, without regard for date, style or medium. Cezanne sits by Sol le Witt, Monet by Richard Long, Matisse by Marlene Dumas, Bonnard by Freud, a Stanley Spencer Resurrection leads on to Fluxus games.

In theory, the hang is organised into four thematic categories. On Level 3, it's broadly still-life and landscape; on Level 5, it's broadly the nude and history - or so the big placards tell you. But these categories are so broad, and so freely interpreted, as to be almost invisible. No doubt they helped the curators keep their bearings. But I hope no one will spend too much time worrying why Carl Andre's bricks are in "History/Memory/Society" and Bridget Riley's stripes are in "Landscape/Matter/Environment". It doesn't matter.

The idea of a non-historical hang is attractive. The visual arts are excessively chronologised. At university, what you study is art history. Modern art is barely conceivable as other than a succesion of groupings and advances. To break or shake this gang mentality is welcome.

A thematic hang is also, in several ways, handy for the gallery. It makes it easier to include more women artists than a strict history-book approach. It disguises the patchiness and imbalances of the Tate's holdings - a huge new building allows more to be seen, but it's still the same collection, strong on Mondrian, strong on Giacommetti, weak on Matisse, weak on Pollock, and with a heavy British weighting.

As you move through the networks of rooms, however, this isn't really what you notice. What you notice is the curating - not on the level of the big broad themes, but in the micro-texture of the hang, in the concepts governing each individual room, and in the stage-craft, the DJ-ing of the sequences of rooms. This is the most intensively curated permanent collection I've ever seen, a triumph of the craft.

The key-note is surprise. The scheme never stays the same. One room will be devoted to a particular subject (war, say), another will have work by a single artist (Tony Cragg, Bruce Naumann), another will focus on a stylistic tendency (primitivism, de Stijl), and another will contain just a single work of art (Leger's film Ballet Mécanique).

Connections are sprung at every corner, an endless game of spot the likeness - from a room of grid-like Mondrians you move to one where the first image is a grid-like Gilbert and George. There are cunning pranks, such as the dimly lit chamber called The Subversive Object, which looks like a collection of Surrealist fetish toys from the 1930s, but then half of them turn out to be neo-Surrealist fetish toys from the 1990s.

Everywhere there are pointed juxtapositions, sometimes sensitive, sometimes crass. There's even a set-piece self-referential joke - a room which still seems to have the builders in, but is in fact a perfect replica, in painted polystyrene, of a roomful of builder's detritus, by the artists Fischli and Weiss.

Perhaps the oddest trick is two little shrine-cum-peepshow cabinets, one containing Degas' Dancer, the other Warhol's Brillo Boxes. I don't quite get it - but I get the general message, that the pace of the stage-craft must not slacken. The rooms lead you on. What new wonder will the next hold? Modern art, it is made clear, shall not for a moment be dull.

Indeed, it is all a wonder. I walked through it first time round in a dream, agog at the constant variety, the maze of vistas, the cornucopia of rooms, the flow and plenty of it. It seemed to be a kind of art-heaven. But then I began to wonder what exactly I was looking at.

It's a spectacle, all right - and one where the most spectacular work, the biggest, brightest, buzziest - which is to say the most recent - makes the going. In this setting, it is very hard for a small painting not to appear as an especially boring kind of video. Tate Modern feels more like a post-1945 or -1960 gallery than a post-1900 one, with the older work included to add historical context to the new, and to look a little defeated. Eventually, 1900 will no longer seem the turning point: classic Modernism will look too classic to be modern and will join the other ancestors in the National Gallery.

It's a spectacle and, above all, an experience. Individual pieces are so superbly integrated into the whole that the display subsumes them. The very deliberate choosing and staging of work means - and this is good - that you notice every single item shown, and notice everything in the context of its staging.

Take one of the most striking adjacencies - a Richard Long stone circle confronts a Monet pond painting. What do we see? A thing on the floor against a thing on the wall; a round against an oblong; found against made, rock against water; and both things in a way about the contemplation of nature - yet in what different ways. The juxtaposition wins all our attention. The two works are compounded into a single piece. They are treated as found objects, picked up by the curators on the streets of art, which, set together, produce something new and interesting and resonant. And sure, they do. But when the Monet was in the National Gallery people used to look at it. Here one isn't meant to do much more than register it - and the Long suffers too (though less).

An art gallery, what is it? Library or theatre? With a library, you return again and again and search, and the only curatorial necessity is that you know where to find things. With theatre, once may well be enough, but the punter absolutely must not go to sleep. It is clear which way Tate Modern leans. It's not a gallery I can easily imagine dropping in on to have a look at a couple of things. It expects a full visit, an outing, with grub and treats. And though I can imagine going again, to get the full measure of this remarkable experience, I'd prefer to go with someone who was seeing it for the first time.

This is the gallery our present art demands. Modern art was born in conflict, opposition, difficulty, offence, and they used to say the museums tamed the art. But this is a museum for art that has tamed itself, become reconciled to being one more contribution to the thing which is now almost everything, the show that must go on, the all-pervading culture of leisure and diversion. Tate Modern points the way ahead. We have seen the future - and I don't call that work.

Tate Modern, Bankside, London E1; open every day; free; special exhibitions £3