Beirut, The Forum, London

The precociously talented Zach Condon and his band, Beirut, aren't ones to tout their wares on the touring circuit. Their last outing, at the Roundhouse in November 2007, was hailed one of the highlights of the year in music, so it's not surprising that, with a huge cult following, tonight's gig sold out within 90 minutes with tickets on Ebay for £70.

New Mexico-born Condon discovered his passion for Balkan folk and Gypsy music when he was a teenage school dropout travelling through Europe. When he arrived in 2006 with Gulag Orkestar, a debut album of Balkan folk-pop, he faced questions of authenticity. But tonight, as confirmation of his dedication to the brassy music ingrained in him, a tattoo of a French horn adorns each of his wrists.

Just 23 years old, the cherubic-faced Condon and his casually dressed band are expert multi-instrumentalists, wielding accordions, trumpets, trombone, maracas, mandolins, ukuleles, bass guitar and double bass in a potent concoction that manages to sound both rambunctiously exuberant and beguilingly emotive. It's hard to imagine that many of this crowd would seek out the music of Balkan folk troubadours, or Mexican mariachi, for that matter, but Beirut manage to make their music so infectiously appealing that each time the three brass players, led by Condon, begin to play, there are spontaneous whoops and applause. There is nothing on the music scene that sounds quite like Beirut.

That's partly thanks to Condon's voice. It has the timbre of Frank Sinatra, with the emotional intensity of Morrissey and a quavering fragility that lends a genuine indie-pop edge, providing a thread throughout the hybrid set of Gypsy, Balkan folk, swing and marching band. While their February release comprises two EPs that have little to link them (one backed by a Mexican funeral band, the second, an electro-pop foray) tonight Beirut weld the genres into a cohesive set of songs. Disconnected on record, "My Wife" and "The Concubine" sound perfectly fitting live.

It's the less cacophonous songs of their earlier material, such as the accordion-driven "Elephant Gun", with delicate mandolin and heartwarming trumpet refrain, that strike the deepest chord tonight. But the venue's sound caters better to the bold brass section than the defined delicate strings of the ukulele or mandolin. The high speed oom-pah of the trumpets is so triumphant that Beirut conjure up the vibe of a carnival, and everyone dances.

The thundering of foot-stamping is one of the most feverishly insistent requests for an encore I've ever encountered. It's rewarded by the band hurtling cheerfully through the joyous racket of Macedonian band Kocani Orkestar's "Siki Siki Baba", Condon injecting yearning emotion into the vocals.

The crowd leaves on a high. If only we could be let into Beirut's world a little more often.