IT'S art innit? Or is it? I confess to causing a disturbance at the new Tate Modern Gallery - that re-ordered power station on the south bank of the Thames at the end of the Millennium Bridge.

That's the bridge that was closed as soon as it opened, because its architect and builders had not allowed for the fact that the large numbers of people walking across it would cause it to sway unacceptably. Now that the bridge is open again, we can all walk across from St Paul's Cathedral to Tate Modern in about five minutes - thus establishing a new shortest distance between the sublime and the ridiculous.

What the Tate needs is a visit from the little boy in Hans Christian Andersen's fairy tale who alone had the courage to announce that the king had no clothes on. In the absence of the little boy, I tried to do his job for him. The so-called "art" in Tate Modern consists mainly of "installations" and an installation is a piece of conceptual art: that is its aesthetic qualities - what aesthetic qualities? - are secondary to the "idea" of it. So, for example, an installationist might conceive of a carpenter's workshop and then construct the same out of polystyrene. There are plenty of examples of this sort in Tate Modern.

Room after room of pretentious junk. There is, for another instance, something that resembles the mess that the builders have left at the back of our church while they have been working here these last six months. You see members of the great British public gawping with dumbstruck admiration at this rubbish. Well, this is where I created my little bit of havoc. I ignored the pile of junk and drew an attendant's notice to a fire-door. "Look," I said in my loudest parsonical voice "at that magnificent installation of a fire-door!"

"Oh no, sir, " said the attendant, swiftly and politely correcting my great ignorance "that's not a work of art: it's a real fire-door."

But I persisted, "Oh do come off it! How can you pretend that a piece of construction so beautiful and finely-wrought is not a work of art?" I began to walk up and down the room, drawing other visitors' attention to the fire-door as I went: "Look at the exquisite line! The way that the handle is fixed deliberately on to one side instead of the other. Sense, if you will, the sheer fire-doorishness of this fire-door!"

In the end, the attendant called for his colleague and they had to restrain me. I protested that what they so rudely called my "making a nuisance of myself" was really my own work of art, my own personal statement of true artistic integrity; my very own installation. I had to leave, but not before I'd thoroughly enjoyed my ten minutes of "artistic freedom"; my finger-pointing, nose-turned-up mockery. I must say, though, that Tate Modern does offer access to one supreme work of art: from the second floor, there is the most spectacular view across the river to where stands St Paul's in all its magnificence.

* Peter Mullen is Rector of St Michael's, Cornhill, in the City of London and Chaplain to the Stock Exchange