Fleet Foxes, Roundhouse, London

Fleet Foxes are doing even better here than back home in the US. This is the first of three consecutive sold-out nights for the Seattle five-piece, who scored two Brit nominations. On the back of their debut release, the EP Sun Giant, and a SXSW appearance, they were hailed the best new band to come out of America, and now their album, which made it into the critics’ best-of lists of last year, has gone platinum.

Yet lead singer and songwriter Robin Pecknold is humble in surveying the deafening ovation that greets their performance. “Thank you so much,” he repeats earnestly to every corner of the crowd, almost bewildered by the long-lasting applause. He has just played the pinnacle of an astounding set, including a gorgeous cover of Judee Sill’s “Crayon Angels”, which masterfully led into an impassioned and compelling “Oliver James”, drawing the listener into its folk-ballad tale of death. Even when unaccompanied by his acoustic guitar, Pecknold’s voice rips through, raw and reverberating as it hollers straight from the West Coast, belying his youthful 22 years.

Their look – plaid shirts and scruffy beards – belies their age, too, further invoking the origins of their pastoral alt folk. There’s a good dose of Crosby, Stills and Nash, English folk and an echo of the ethereal rustic world that the alt-folkers Midlake captured in their album The Trials of Van Occupanther, especially in “Ragged Wood”.

Even when Pecknold performs a traditional Scottish folk song, he imprints it with the band’s strong sense of musical identity. The Roundhouse’s acoustics are perfect for allowing his vocals to soar in this unamplified version, which reduces the excitable crowd to silence. They give as good as they get to the hecklers, too: to see the sharp Pecknold and drummer Josh Tillman compete for witty banter is entertaining in itself.

Their opener, “Sun Giant”, is a courageous a cappella four-part vocal harmony, immediately showcasing their sunkissed baroque Americana. When they merge into “Sun It Rises”, they evoke the warmth of its subject with the blissful build-up of guitars, and the effective use of a bow on electric guitar. It’s their magnificent harmonies that set them aside live, lifting songs into a euphoric surge, but with a haunting, mysterious tone that never allows them to be twee. Leading from one track to the next could be the movements in a symphony.

But there’s a pleasingly breezy lightness to their music, too. “English House” skips along with its complex, interweaving guitar and high-pitch harmonies. And while it’s all meticulously orchestrated, they are not afraid to improvise. In response to a request for his favourite Bob Dylan song, Pecknold plays “It Aint Me, Babe”. “Mykonos” and the glorious “White Winter Hymnal” both get huge applause.

Fleet Foxes are on a ladder that keeps going up; their potential is sky-high.