Grizzly Bear in London last month; (left to right) Chris Taylor, Dan Rossen, Ed Drosle and Christopher Bear

Pop is no longer scared of classical music. In the years of BBC’s Top of the Pops, for instance, pulchritudinous cellists and violinists may have lent the pop groups gravitas, but they appeared only in the background, usually uncredited. All that has changed, and bands such as Grizzly Bear go much farther than making free with the odd classical motif in their songs.

The brainy, indie-rock four-piece from Brooklyn are now winning wider mainstream recognition for their third album, Vecktatimest, having previously been darlings of the blogosphere. In March, they played jointly with the Brooklyn Philharmonic and co-headlined a show with the LA Philharmonic; next month they face their biggest classical collaboration so far: a date with the London Symphony Orchestra in the Barbican Hall.

“We’ll be more familiar with the synergy of playing with that many people on stage,” says Ed Droste, the band’s founder, when I meet him and his main co-vocalist, Daniel Rossen, the day after a more conventional gig at London’s Koko in August. “The great thing is that they’re providing us with more rehearsal time. At the Brooklyn Phil, we got three hours – barely enough time to run through the set once or twice; this time, it’s seven or eight.”

That set will largely be derived from Vecktatimest, which peaked at No 8 in the American chart and almost broke into the UK’s Top 20. Named after an uninhabited island off Cape Cod, the album is a woozy yet exacting mash-up of doo-wop harmonies and Astral Weeks-ish jazz-folk, sharpened with a latent art-rock attack – and it’s likely to be many critics’ record of the year. If its textures are almost too intricate and overlapping to replicate faithfully in a rock venue, such details should be drawn out, and even accentuated, in a classical context.

What Grizzly Bear played with the Brooklyn Phil was only partially orchestrated; the songs with the LSO will be much more full-on, and that, says Droste, is down to giving their arranger, Nico Muhly, “free rein”. Muhly, a talented young composer plugged in to the hip New York music scene, has previously worked with Antony and the Johnsons; this led to an engagement with the LSO last year. On Veckatimest he wrote vocal arrangements on two songs and strings for three – although, as Rossen says: “Nico’s stuff got folded in with everything else, and it’s very hard to pick it out sometimes.”

At the time of our interview, Droste hadn’t heard Muhly’s latest orchestral arrangements, and while with the Brooklyn Phil the aim was for the songs to be “in the same vein as the album”, with the LSO “a couple of them might be completely turned over”. Therefore, this live version of the album “could be a completely different beast,” he adds.

The band members are all skilled musicians. Rossen, the guitarist, attended a music academy as a teenager; while the bassist Chris Taylor, who also plays flute and bass clarinet, and the drummer, who is actually called Christopher Bear, both studied jazz at New York University – “Bear is still really a jazz drummer,” says Rossen. “He can be wholly bombastic or really restrained. That’s probably why he’s so great.” It’s only Droste who doesn’t read music, even though his grandfather was a Harvard music professor and his mother taught music to schoolchildren.

In their turn, the LSO hopes the classical experience rubs off on Grizzly Bear fans. The orchestra’s managing director, Kathryn McDowell, says: “Projects like this open up a different sort of audience for the LSO, and we’re a partner in the whole collaboration, not just a backing band.”

This last remark touches a nerve. When the wonderful British group Elbow performed with the equally wonderful Hallé Orchestra at the Manchester festival in July, the show was hailed as a celebration of two home-town heroes, but some critics felt the Hallé were trapped in that “backing band” role. McDowell thinks that mistake won’t be made on this occasion.

“When you approach such collaborations,” she says, “you have to think, ‘What’s the music actually like? Is it original, is there freshness to it?’ And how are the arrangements for orchestra going to be done?’ Because it’s the aptitude of the arranger that really makes these collaborations come alive: Nico has that.”

Rossen describes Muhly’s style as “very Scandinavian, very angular and very icy, with different pieces going off in different directions”, and the Philip Glass protégé will be on stage with Grizzly Bear “translating” the LSO conductor’s promptings. That the conductor is André de Ridder bodes well for the performance: the German has worked on Monkey: Journey to the West, Damon Albarn from Blur’s acclaimed Chinese-inspired music-theatre piece.

McDowell would like to see the LSO collaborate in this way at least once a season. As it happens, the Barbican – thanks to the foresight of its contemporary music programmer Bryn Ormrod – is hosting a show by the Danish group Efterklang and the Britten Sinfonia a few days before. And Grizzly Bear and Efterklang are not alone in putting out feelers into the classical world. Earlier next month, Arcade Fire’s six-piece instrumental offshoot, Bell Orchestre, releases its sophomore album, As Seen Through Windows, and so does Sufjan Stevens his neo-Gershwinesque homage to the Brooklyn-Queens Expressway, The BQE.

“I don’t think categories matter now, as they may have done 20 years ago,” says McDowell. “There’s a coming together of disciplines. At the avant-garde end of contemporary composition and the advanced end of pop culture, there’s no real distinction – they’re all highly inventive artists working at the top of their game.”

Grizzly Bear and the LSO, Barbican, London, October 31. www.barbican.org.uk. Efterklang release ‘Performing Parades’ (with the Danish National Chamber Orchestra) on October 19 on the Leaf Label – their Barbican concert with the Britten Sinfonia is on October 28; ‘As Seen Through Windows’, by Bell Orchestre, is out on October 5 on Arts & Crafts; Sufjan Stevens’s ‘The BQE’ is out on October 20 on Asthmatic Kitty