Adam Levine watches as his girlfriend is strapped into a ball-shaped metal frame that will soon be hurled skywards on elasticated ropes. Yet rather than running out to rescue her, the Maroon 5 singer and lyricist starts to giggle. “That’s f****** insane,” breathes the bassist Mickey Madden, shaking his head and grimacing as Levine’s other half is flung into the air. Her screams carry the short distance across the Del Mar Showgrounds and through the open window of the Portakabin that Maroon 5 are using as a dressing room before their show tonight at the San Diego County Fair.

Their first two singles, the Zeppelin-esque Harder to Breathe and the piano-led soulful strut of This Love, remain ubiquitous on British radio. Their October tour here is already almost sold out, and music press snobs have started to give them a kicking, but the public has taken this unassuming quintet to their hearts. And today, it’s easy to see why.

The County Fair is a mix of school fête, farm open day and rock festival. In the programme, Maroon 5’s picture nestles at the foot of page 15, a higher billing than the Pet Rock competition or the Pro-Am Bull Riding contest, but a few pages down from the greeting from Governor Schwarzenegger that welcomes us to “the finest agricultural fair in the nation”. The month-long event is a bewildering assortment of livestock competitions, children’s art, vegetable shows and fairground entertainment. Oh, and foodstuffs of a variety that your sheltered correspondent had assumed existed only in the imagination of Homer Simpson: Pizza on a Stick, anyone? Maroon 5 could hardly seem more out of place, but to them, this is just another date.

“There’s no way this is the weirdest gig we’ve played,” Levine shrugs. “That, I think, took place one very rainy night at Six Flags in New Jersey.”

“It was in an amusement park,” says Madden. “There were about seven people watching us, and as we started to play the most horrible thunderstorm set in. After about four songs we had to stop.”

“We all got really, really stoned,” recalls the group’s keyboard player, Jesse Carmichael, with a grin.

“So we played this improvised 20-minute version of our song The Sun,” says the guitarist James Valentine. “And, at the climax of the improv section, the heavens opened.”

“Those seven people really liked us,” Levine sighs. “But you know who didn’t? God.”

“Yeah,” Valentine nods sagely. “His review was in.”

Even given the Almighty’s wrath, the San Diego County Fair does not seem like a show that Maroon 5 need to do. This is a band (Levine, Ryan Dusick, Valentine, Madden and Carmichael, left) who are riding on the crest of a wave, yet teetering on the brink of exhausted breakdown — a gig at a racetrack with a girlfriend-threatening funfair on site seems neither advisable nor necessary.

Last night, Maroon 5 recorded a performance of their third single, She Will Be Loved, for the Tonight Show in their home town of Los Angeles. From there, they headed off with Tobey Maguire to the premiere of Spider-Man 2 (they have a song on the soundtrack album) and drank bourbon with the stars.

“But it’s the diversity that I think is cool,” Levine argues. “Last night was really fun. I got drunk and had a fun time, but it’s over now.”

“I don’t think there’s any sense of anyone in this band resting on any laurels,” agrees Madden by way of explanation for their presence today. “I don’t feel as if we’ve earned it. Right now it feels like we’re in the middle of this tour for our first record, and until that’s over we probably won’t stop and process it. There’s just so much momentum.”

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