A couple of hours before the band are due on stage at London's Electric Ballroom, Glaswegian power-punk trio the Fratellis are discussing the mayhem they're encountering on the road. Specifically, an incident in Arbroath. Corkscrew-haired frontman Jon Fratelli (they adopt a Ramones-like gang identity, though bassist Barry was born with the surname) was suffering from food poisoning and politely informed the crowd he could not carry on, at which point, he says, "All hell broke loose."
"The crowd suddenly started killing each other," he sighs. "Bottles were getting smashed over people's heads. Bodies flying everywhere. It was pure chaos." Drummer Mince Fratelli took refuge in the women's toilets, while the ill and bewildered singer was escorted from the dressing room by a policeman.
You won't see this sort of thing in stadiums, where crowd barriers and security teams hold sway, but small to medium-sized venues are increasingly having to deal with unruly audiences, flying instruments and - especially - stage invasions. "It does seem no small gig is complete without one," says Conor McNicholas, the editor of NME. "They used to happen rarely and would be talked about as legendary gigs. But lately you go to some gigs and if there hasn't been a stage invasion, you feel like you've missed out."
It's happening because, as indie and rock have replaced pop as the teenager's preferred music, audiences are getting more enthusiastic. But equally, there's a sense of a backlash against two years of endless middle-of-the-road, piano-led rock bands and singer-songwritery gigs where nothing tends to happen. "Kids are looking for more excitement," says McNicholas, "and that means wilder live gigs." Beneath his Bolan-Dylan homage of a haircut, Jon Fratelli is quietly but spikily dismissive of "all these singer-songwriters who think it started with David Gray".
The growth in stage invasions can be traced back to the Libertines, who took the Clash's "anyone can do it" philosophy a step further by declaring there was no difference between a band and the audience, and that both deserved to be on stage. One of the Libertines' final gigs - at the Forum in north London - saw one of the biggest invasions ever of a British stage, commemorated in the video for the Can't Stand Me Now single.
However, the trend has rocketed lately along with the emergence of a new group of rabble-rousers, led by Arctic Monkeys: a clutch of bands making rowdy music that lends itself to crowd participation.
The Fratellis fit the profile. Their songs are short, exuberant blasts that cram the Hamburg-era Beatles, rockabilly and the Clash into a deranged glam stomp that is the polar opposite of the studied art rock fellow Glaswegians Franz Ferdinand. Fratellis choruses have a swaggering, crowd-swaying, intoxicating quality: it's almost music hall. Take away the guitars and songs such as Chelsea Dagger sound as if they could be mass singalongs around the piano in a 19th-century pub. "To me, they sound like your cousin's band playing 1970s covers at a Glaswegian wedding," says McNicholas. "It's unpretentious, completely beer-fuelled and fantastic."
NME have dubbed the Fratellis "the rowdiest band in Britain", but there's plenty of competition. Leeds-based band the Pigeon Detectives have built an enormous live following and Arctic Monkeys-like word-of-mouth buzz on the back of two singles and a reputation for stage invasions. At an average age of just 22, they arrive for their first ever newspaper interview in a graffiti-covered Transit van, and are soon comparing scars.
"That's from where Ollie [Main, the guitarist] whacked me in the face with his guitar and knocked me off my feet," explains Matthew Bowman, another curly-topped vocalist, whose building-site bellow sits atop his band's collision of the Buzzcocks and the Smiths and who - when the mood takes him - invites audiences on stage.
For his sins, Main has similarly been "taken out a few times" by Bowman's demented mic-twirling, a feature of their live gigs that isn't quite as professional as Roger Daltrey's. When the Pigeon Detectives recently played in Southampton, the microphone came apart from the lead and flew from the stage, hitting the back wall with a thud. "Two little kids brought it back," grins Bowman. "Which was helpful." At another gig, in Sheffield, the band were alarmed to discover that the stage invaders were dismantling the drum kit and walking away with it.
Welsh pop-punk band the Automatic are another band used to curious behaviour at live gigs. During their T in the Park set, a number of punters took unusual steps to avoid festival security. "Five or six of them climbed up the scaffolding," explains singer Rob Hawkins. "One of them was having dry sex with the tentpole. There were more people watching him than us."
The sense of mania surrounding these bands is reflected in the speed of their rise in popularity. The Fratellis met while working on Barry's father's fairground - the Fratelli name was painted on the waltzers - but after a whirlwind period in which their demo tape flew around London, they hit the top 20 with their second single in June. The Pigeon Detectives knew each other from school but have gone from being "rubbish" to skirting the fringes of the charts inside a year. They have been courted by major labels, but are currently turning them down after "thanking them for all the meals".
The Automatic, another band of schoolmates, had not played outside Wales before they were picked up by B-Unique/ Polydor. With their signature-tune single Monster having spent the summer in the top 10, they have barely had a day off, but they're having fun. After a recent gig in Brighton, the band stripped to their boxers and charged into the sea, followed by 200 fans.
Audience participation is being fuelled by the internet, especially the rise of MySpace. Because the website allows fans to discover bands much earlier, audiences have started to feel as if they are part of a band's extended family.
"Previously an A&R man would slog up to Darlington, see a band, sign them and no one would see them for two years," says McNicholas. "But MySpace has meant that the kids own a bit of the band from day one." Recently, the Pigeon Detectives have noticed that the first five rows at gigs now know all the lyrics from hearing songs on MySpace. A recent Leeds gig - where the stage invasion was led by none other than celebrity fan Carl Barat, of Dirty Pretty Things and formerly the Libertines - saw a rush of internet forum activity proclaiming "the wildest gig in Leeds this year".
The industry is not slow in turning that enthusiasm into "street teams" - armies of followers who help market a band by talking about them and posting messages on forums. It can seem a bit like the Moonies, but McNicholas argues it's "part of that glazed eye thing - you feel like you're helping your heroes. The natural conclusion is to get up on their stage."
But there is more to these bands than wild gigs. The Fratellis' forthcoming Costello Music contains acutely observed tales of semi-mythical Glaswegian characters such as Vince the Loveable Stoner, or the pill-popping female in For the Girl who was "into the Stones when I was into the Roses". On record, their basic three-piece sound is augmented by horns and other textures. They even have a couple of slow songs - although Barry observes that audiences "crowd-surf during the slowies".
The Pigeon Detectives are wary of slow numbers because "we'd sound a bit like Keane", but the breakneck You Know I Love You is a witheringly observed pastiche of "things lads say to get girls into bed".
The Automatic's songbook was composed in their "gift-shop market town" of Cowbridge, where they longed to escape the mentality where "you get your A-levels, go to university and set yourself up", explains Rob Hawkins. Now, it thrills him that their most riotous audiences have been in similar small towns, such as Northampton and Tunbridge Wells. "We seem to be connecting with kids who are in a similar position to what we were," he says. "Anyone who feels like they're being forced to conform."
Where it will all lead is anyone's guess, but venues are starting to realise that they have to adapt to their audiences, not demand concert-hall reverence. In particular, stage invasions seem to be more accepted by security teams in many clubs, who now prefer to lead fans calmly off to the wings, rather than thrusting them immediately back into the throng with an aggressive sneer.
The probable outcome, however, is the Beatlemania response. Just as the Beatles got sick of the screaming, which led them to the admittedly extreme response of retiring from live performance in 1966, so these bands will likely reach a point where they'll tire of mayhem themselves, and want to be recognised for their music rather than their fans' rowdiness.
"We know it can't carry on like this forever," muses head Pigeon Rob Bowman. "Sooner or later people will go back to just listening to the songs." But for now it's fun and exciting and - so far, as Bowman points out - "the great thing is nobody's getting hurt".
He pauses, and rubs his bruises. "Well, only us."
· The Fratellis' first album, Costello Music, is released on September 11 on Island. The Automatic's next single, Recover, is released by B-Unique on September 18. The Pigeon Detectives release a third single in October.
