MGMT, The Forum, London

Of all the Brooklyn exports to the British music scene, MGMT span the most musical generations with their blend of Sixties synth-fuelled psychedelia, Bowie influences and rock. That tickets for tonight sold out in June and touts are selling them for £100 outside is testimony to their meteoric rise.

It's hard to know what to expect from an MGMT show, but wacky costumes and psychedelic twists are likely. Tonight is no exception; the stage is filled with huge cardboard cut-outs of cacti, while the three-piece live band are dressed as Native Americans. Guitarist/singer Andrew VanWyngarden could have walked out of a meeting with Cromwell. Ben Goldwasser on keyboards looks the quintessential indie kid with his white Ray-Bans – but there's a feather tucked into his headband.

It's been suggested that MGMT's live shows lack the punch of their melody-filled debut album Oracular Spectacular. Live, they are certainly more prog-rock than pop-rock. Their opening songs here, especially the instrumental-heavy "Fourth Dimensional Transition", drift into extended, non-focused instrumentation. It's a relief when they slide into "Pieces of What", an intoxicating melody showcasing VanWyngarden's vocals. Even in "Weekend Wars", in which Goldwasser's fingers scuttle across the keys, they can't resist a drawn-out instrumental ending.

But the crowd erupts at the first notes to "Time To Pretend", their sardonic song about the trappings of growing fame ("Let's make some music, make some money, find some models for wives") is euphoric pop. You'd have thought that, with a party song like this, MGMT would be loud and exuberant, but they hold a mellow ambience. To follow up with "Metanoia", which can rival any Flaming Lips song for tempo changes and goes on for at least 10 minutes, is odd. As VanWyngarden noodles with his back to the crowd, it's not long before some fans start singing the catchy riff to "Kids", showing the need for a bit more punch and melody.

A lively "Electric Feel" lives up to all its promise, trumped only by the anthem of the year, the anticipated "Kids", greeted by a sea of bobbing hands. The duo are in their element now, standing side by side and singing. If they'd performed to this pace through the set they'd have brought the place down.

"So this is really sad," says VanWyngarden. "It's our second-to-last show here for a very long time." It's also likely to be the last time MGMT play such an intimate venue. They're going far.