Enjoy our new City Hall - while it lasts

London's new City Hall is elegant and exciting. Opened on Tuesday by the Queen, it boasts the confidence for which London government has been yearning for two decades. I expected a monstrosity. I was wrong. For the most part, this is a place of beauty. Will it stay that way?

Politics did its best to ruin the first day. As the Mayor, Ken Livingstone, welcomed the monarch to the building, his beloved Labour Party was rejecting his application for resumed membership. He had to complete his royal duties spitting with fury.

The deputy mayor, Nicky Gavron, duly announced that she would challenge his re-election in two years' time. Meanwhile, the Mayor's transport chief was back in the High Court, still suing central Government, and his cronies were upstairs counting their salaries. What a mess.

I celebrated the occasion elsewhere, with a certain old lady shedding a tear two miles upstream. Ever since the crass Mr Livingstone brought the Greater London Council down about his ears in 1985, the core of County Hall has stood echoing and empty. The desks are still in place. Parlours and ballrooms stand dusty and forlorn. Banners, coats of arms and names of council leaders gaze down from dignified walls. Outside the windows, the grotesque London Eye clanks and whirs in humiliating proximity. Sic transit gloria.

The lady's successor could not be more different. It is a City slicker. Designed, as is half London these days, by Norman Foster, it has been called a testicle, an egg on steroids, Darth Vader's helmet, a mailed fist, badly stacked soup plates and "one bum in Y-fronts". The bizarre shape would look less odd if located, as intended, forward on the edge of the water. An idiot planner ruled this out of order.

Some things are wrong. The cladding of steel and glass is cold and hostile. In anything but bright sunlight the exterior is gunmetal grey. The minimalist entrance is also disappointing for a great municipal building. Since the site is hard to reach by public or private transport, this does not seem a home of democracy, more the austere headquarters of a regulatory agency.

These faults are mitigated by two brilliant architectural coups. One is the riverside setting. Downstream are gardens raised and landscaped so that Tower Bridge, the Tower of London and the City seem to float on grass over the river. Immediately upstream is an amphitheatre embracing a sunken cafeteria. This "scoop" is well-conceived and friendly, spilling down from the river walk.

More remarkable is the interior, one of the most startling spaces inside any modern London building. The assembly hall forms a conventional atrium, but above it coils and spins a spiral walkway. It rises 10 floors and gradually tilts backwards, drawing the eye as if to infinity.

It is asymmetrical and disorienting, with no shape the same as any other. To wander its upper levels is to be lost in swirling decks and planes, always with London as a backdrop. From the summit is a viewing gallery with a 360-degree look-out, offering one of the best panoramas in London. And it is all open to the public and free.

But see it fast. The building now stands isolated in a small park. It does not belong to Londoners, being built by private developers and offered to the Government on a 25-year minimal rent. This was a planning bribe to allow a huge commercial development, which will eventually encircle the City Hall on all sides.

This is the site that once saw a competition between the American architect, Philip Johnson, and Britain's John Simpson. That opportunity fell victim to Chris Patten's indecision as environment secretary. Now a dreary forest of the Mayor's skyscrapers is proposed, between his office and Tower Bridge. Behind and upstream, yet more Norman Foster boxes and glass walls are starting to rise.

Desperate Georgian and Victorians buildings are struggling to retain some character along Tooley Street to the rear, but with the Mayor and his anti-conservationist adviser, Lord Rogers, glaring down on them, they must be doomed relics of the Ice Age.

It is right that this reach of the Thames should be densely packed. This was the old Pool of London, once crammed with warehouses. But the blitzing of anything remotely old on these sites is a tragic rerun of the mistakes of the 1960s and 1970s. The inability of architects to incorporate old with new is unimaginative (or merely unprofitable). The aversion to London materials such as stone and brick is equally so. Such materials age and colour and enliven the street scene.

Lord Foster's attachment to glass cladding recalls the poured concrete beloved of the commercial architect his work increasingly resembles, the late Richard Seifert. Neither glass nor concrete suit the London climate.

In 25 years the Great London Authority's preferential rent on City Hall will soar. The building may well be regarded as too expensive and, like County Hall, be vacated for somewhere else. This assumes that, before then, the Mayor does not so enrage central Government as to bring ministers marching back and pulling down the shutters.

Every good building should be adaptable. It should be reusable. County Hall was built for government, but most of it is now flats, hotels, a conference centre and an aquarium. The new City Hall would be hopeless for most such uses, except perhaps an aquarium.

When they dump Mr Livingstone a second time, the Government could simply turn on the taps and fill the place with water, turning it into a giant goldfish bowl. City workers gazing from their Foster windows would see sharks and piranhas cruising 10 storeys up before their eyes. It would be Europe's supreme Surrealist joke.

But that is for the future. For the present, let us all enjoy it.