Camden Roundhouse was cleared during the AMP Sounds closing party featuring DJ Annie Mac, Wretch 32 and Disclosure tonight.
Eyewitness video from the scene shows crowds of revellers gathered on the street outside the venue in north-west London.
Journalist Hayley Minn, who is at the gig, wrote on Twitter: "We’ve all just been evacuated from Roundhouse Camden. Anyone know why? #ampsounds @AnnieMac."
She told told Daily Star Online the building was evacuated after an alarm went off.
"Basically, alarm went off and everyone was evacuated for about 15/20 minutes," she said.
WIKIPEDIAEVACUATED: The Camden Roundhouse has been cleared during an Annie Mac Presents show
Another gig-goer added: "What’s going on in London? My sister's there and been evacuated from Roundhouse, no news or updates on what’s happening?"
Music fans have since been allowed to return to the venue after security gave the all-clear.
The nature of the energency – and why the alarm went off – is not yet clear but London Fire Brigade has not been called to the venue.
Someone allegedly triggered the alarm while rapper Ghetts was performing on stage, MirrorOnline reports.
HAYLEY MINNON THE STREETS: Crowds of music fans gather outside the venue after the evacuation
Tweeting about the incident, Annie Mac posted: “The party is back on after a slight blip - basically @THEREALGHETTS was too good and we had to evacuate the building @Amp"
Annie Mac is performing alongside Ghetts, Mabel and Mist & Steel Banglez among others tonight at the charity fundraising event for The Roundhouse Charity.
A description of the event reads: "DJ and Radio Presenter Annie Mac brings the AMP Sounds closing party to the Roundhouse for a special night that will celebrate the very best of the album’s past, present and future."