There's little point attending a Green Day show expecting punk rock. The California three-piece – swollen to six for performance purposes – emerged from the west-coast hardcore scene, but there's a reason it is they who are touring the world's arenas, rather than, say, fellow San Franciscans NOFX. That becomes apparent here within the first five minutes, when miniature frontman Billie Joe Armstrong engages the crowd in the first of dozens of call-and-response rituals: this isn't a punk gig at all. In fact, to those of a certain age, it resembles nothing so much as an epically extended edition of the ancient BBC kids' show Crackerjack.
As well as leading the singalongs, Armstrong sprays the front rows with hoses; shoots T-shirts into the crowd with an air gun; careens about the stage as pyrotechnics go off around him, like a demented pantomime Buttons; and drags teenagers up to be lead singer for a verse. This leave-no-piece-of-stagecraft-unturned approach minimises the main problem of the O2: the fact that those at the back can feel they are in a different postcode to the performers.
Green Day are breathlessly keen to remind everyone that, £20 beanie hats at the merchandise stall notwithstanding, they remain true to their founding ethos after 21 years together. Every so often, they return to their early material for a couple of minutes of rama-lama guitar-bashing, and during King for a Day, the screens flash up flyers for gigs from hardcore's first wave – bands such as Bad Brains, Black Flag, MDC and Gang Green. But the true nature of the gig lies in the next song, a version of the Isley Brothers' Shout! that also incorporates Satisfaction, Teenage Kicks, I'll Be There and I Fought the Law.
It would be easy to mock this as plastic punk, but Green Day have managed something the Clash never could: they have used their huge popularity as a Trojan horse to smuggle politics into the charts. American Idiot, Holiday and Know Your Enemy are borne in on riffs huge enough to raze whole cities to the ground, and the fact that 20,000 people holler every word of American Idiot's rejection of a "redneck agenda" is both exhilarating and encouraging. There's no hint of pomposity, no preaching to the crowd; just the hope they might be taking notice of what they're singing along to. It's all perfectly pitched, and unexpectedly fantastic.
