Their hardcore fans clutter the front of the stage and scream as though the world is being upended. The five boys walk beautifully - there are no dance routines here, this is a Gen Y boyband, thank you - and bounce around like an enthusiastic Top Shop advert, apart from Harry Styles's crotch grabbing, which lowers the tone just a little.
Yesterday, though, swaps boys for men - Kings of Leon appear on the Main Stage just as a hailstorm grips Glasgow Green. Besides an occasional dabbing of brow by lead singer Caleb Followill you would never know the weather was inclement. They play a rousing mixture of the old and new, and during Fans Chris Martin - "the second best singer here today," says Caleb - appears onstage to play his guitar. It's the lovely thing about festivals, the chance for surprise guest appearances and hybrid pop performances, though it feels a little pointless, Martin strumming away beside the Followills.
Back to Saturday and Pharrell Williams is a pop delight. With a collection of highly limber dancers he reels them off - Lap Dance, Drop It Like It's Hot, Blurred Lines, a quick cover of Gwen Stefani's Hollaback Girl, a stonking version of Get Lucky and, of course, Happy.
Matching Pharrell's upbeat good humour is Lily Allen. "I've got a night off from my kids," she says, raising a glass, "And I'm going to get drizunk." With a deep tan and sparkling gold top, Allen carries the crowd along with her on a swell of positivity, not least during Smile. Her performance is a vibrant affair, catching the last rays of the late afternoon sun.
Lorde, a 17-year-old Kiwi with an excess of curls and lip, has the In New Music We Trust stage sewn up. Ribs, with its dance beats, and Tennis Courts knock your socks off. Her audience is on tenterhooks for Royals, which arrives second-last in the set and is not a disappointment. Snapping percussion, tight but relaxed vocals and the kind of delivery that suggests a performer not to be messed with.
Hometown energy was saved for Calvin Harris, again on the main stage, but there was plenty left for Coldplay, who closed Saturday night with fireworks, bagpipes and acres of bonhomie.
Radio 1 touted the weekend as the "best line-up on the planet". There's maybe an obscure music festival in the wilds of Kazakhstan banging out the tunes but, failing that, the BBC is absolutely right.