Howe Gelb & Friends (Upside Down Home), Barbican, London

At the final count, when all the performers had been invited back at the end of the night for what our host, the Tucson-based songwriter Howe Gelb, called a "grand ole opry", there were 16 people on stage. These included Lambchop's Kurt Wagner, the multi-instrumentalist and producer John Parish, Vic Chesnutt, Mark Linkous from Sparklehorse, Calexico's John Convertino and Joey Burns, the former Lemonhead Evan Dando and, starriest of all, PJ Harvey. Then there was the mysterious woman with the Hallowe'en face-paint whose tasks included laying flowers around the stage and taking photos of the audience. Sometimes, even musicians need souvenirs.

It's hard to imagine this second instalment of the Beyond Nashville festival being bettered in the next week. This has to be the first time the UK has seen so many alt-country luminaries gathered under one roof. What's more, it's not often that an audience will sit through three and a half hours of music without complaint.

Howe Gelb is one of the more enigmatic figures in this exciting if ill-defined genre. By no means a household name, he has been the driving force behind a number of bands, from Giant Sand and OP8 to the French-Arizonian outfit ABBC, and, over the last two decades, has put out around 30 albums.

The first ramshackle set came from Gelb himself, his weathered baritone beautifully set alongside Giant Sand's beautiful instrumentals – piano, violins, double bass and all manner of percussive twitching. PJ Harvey, introduced by Gelb simply as "our friend from the West Country", emerged to play a raucous cover of "Johnny Hit and Run Pauline", and later joined Mark Linkous, hunched and grumpy, for a magical version of Sparklehorse's "Homecoming Queen".

A cheer went up as Evan Dando shuffled on to play a pared-down, acoustic version of the Lemonheads' "My Drug Buddy". This was followed by some pensive new material, all of which bodes well for next year's solo album. Vic Chesnutt's excitement at being invited was palpable. "I'm busting my buttons over here," he laughed before performing a terrific duet with Kurt Wagner.

Despite the lofty surroundings, the atmosphere was cosy, as if all 2,000 of us were at a rehearsal rather than a performance. Keen not to miss a thing, one man in the audience shouted, "Have I got time to get a beer?" "I couldn't guarantee it," replied our smiling host.

There were countless great moments – John Parish's intoxicating post-rock instrumentals, a lively jazz encore performed by Gelb, John Convertino and Joey Burns, and Gelb's many wardrobe changes, most of which took place on stage rather than off. What a guy.

Beyond Nashville runs until 12 November, box office: 020-7638 8891