I instinctively shudder at the idea of "perfect cities": give me a rent-free apartment in Le Corbusier's totalitarian Ville Radieuse or St. Augustine's City of God, and I'd politely decline it. I dream, rather, of living in a city like Dickens's London, that marvellous labyrinth of dark alleys, secrets, surprises, extreme economic inequalities, coincidences, possibilities, where every kind of human imperfection and eccentricity finds its niche.

Once, not long ago, people moved to cities to enjoy a degree of freedom and privacy that was denied them by the lace-curtain police of the village and the small town. Only in the city could they find the necessary anonymity in which to reinvent themselves: the city was the natural habitat of the immigrant, the artist, the criminal, the social climber, the oddball, the outcast, and its energy arose from containing this multitude of conflicting ambitions, all set free by the enormous, unconfined, largely unregulated honeycomb-structure of the city itself.

Loving cities, I'm appalled by what's been happening to them in the last six years since 9/11, at least in both Britain and the United States, where the threat of international terrorism (which is at once real and concocted) has led to a vision of the city as a place where all of human life should be open to official inspection. The new architecture is everywhere around us: blast walls, Jersey barriers, concrete bollards to deter car-bombers, walk-through magnetometers at the entrances of buildings, high-intensity street lighting (for security abhors shadows), checkpoints, razor wire, BioWatch air-sniffers, armoured glass, and enough surveillance cameras to watch and map every journey made by the individual citizen.

The security city, currently the darling of governments, aspires to the condition of Jeremy Bentham's 1787 Panopticon-a massive concentric prison in which, because everything is readily observable from all quarters, detainees regulate their own behaviour under the eye of a single, central, symbolically conspicuous jailer. The Panopticon was designed to enforce obedient conformity on the part of its miserable inhabitants. In London, New York, Washington D.C., and Seattle, I see the unquiet ghost of Bentham (whose mummified corpse sits today in a glass cabinet at University College, London) hard at work, making visible what used to be hidden, robbing us of privacies once taken for granted, and systematically frightening us in order to justify this radical erosion of our liberties-cue the colour-coded alert system and the anti-terrorist "exercise" which is the 21st century's unique contribution to the long history of street theatre.

Stand in line. No joking with security personnel. Place your toiletries in a one-quart transparent plastic bag. Take off your shoes. Empty your pockets... The airport security area is the model envisaged by the mayors and Homeland Security chieftains who are spending billions of pounds and dollars in pursuit of their own idea of the perfect city. "Citizen safety" is their mantra, as if we should be grateful to see our taxes spent on gutting the city of its magic and mystery.

The first effect of such measures is to terrorise the underclass-the homeless, illegal immigrants, junkies, prostitutes and beggars. But we who have homes and jobs are victims too, for we're fast losing the chiaroscuro play of dark and light, secrets and revelations, the inimitable diversity of people and experience, that once made city life an inexhaustible source of daily excitements and surprises. Remember Henry James: "London is, on the whole the most possible form of life. I take it as an artist and a bachelor; as one who has the passion of observation and whose business is the study of human life. It is the biggest aggregation of human life -- the most complete compendium of the world."

Where would Henry James go now? Certainly not to Ken Livingstone's London or Mayor Bloomberg's New York, where over-regulation, promiscuous surveillance, and an obsession with security are flattening both cities into monochrome. It's hard to imagine the ever-scrupulous Mr James setting foot in Bombay, or Mexico City, or Cairo, but those cities still retain the brimming vitality, the whiff of danger along with that of raw sewage, the human variety of vagrants living cheek-by-jowl with millionaires, the poorly lit streets and pools of deep shadow, that gave nineteenth century London its extraordinary hold on the imaginations of artists and writers throughout the world.

I'd dearly like to interrogate a bunch of western urban planners and security advisers, and find out which metropolitan city figures most prominently in their nightmares as the worst of the worst. That would be a place seriously worth visiting, for in the matter of cities, it's in gross imperfection that perfection lies.