The setting seemed incongruous on paper, yet on the night, the placing of Britain's premier electro-pop pair in the Tower of London feels appropriate. Gleaming jewels, men in funny costumes - and that's just the Pet Shop Boys' show, which proves a splendid opening act for a two-week festival.

It's an outdoor concert and the night is full of stars, but they're mainly watching the show, with the preponderance of shaven-headed Pimm's drinkers creating the illusion of being at London's biggest civil ceremony.

The duo open with Psychological, and by the second song, Left to My Own Devices, the revellers have already taken to the aisles and blocked the view - where's a Beefeater when you need one? On stage, Neil Tennant and Chris Lowe are surrounded by dancing doppelgangers who pirouette and preen in the same costumes as their paymasters, whether it's top hat and tails or army fatigues. Tennant's supers then change into golden cowboy outfits to perform Where the Streets Have No Name, and it's like watching Brokeback Mountain as re-enacted by Village People. Their ode to 1980s profligacy, Shopping, takes on sinister new depths in the medieval surroundings. "There's a big bang in the City, we're all on the make" sings Tennant, the Gherkin building looming beyond him. When the singer starts beaming from ear to ear, it appears one of the imposters has borrowed his mic, but no, that really is Neil up there.

Fortunately, the pop princes choose not to guard their own crown jewels too closely, so as well as new songs we get all the old hits, including a magnificent encore of It's a Sin. As the crowds ebb away, a lone pavement preacher tries in vain to convert the masses who have spent the night wholly entertained by wickedness.

· At Liverpool Pops on Sunday. Box office: 0870 151 4000