Ten fire engines and around 70 firefighters were called to a fire at a leisure centre on Norman Street in Islington.
A sauna was destroyed by fire and part of the basement was also damaged by the blaze. There were no reports of any injuries.
Station Commander David Reed, who was at the scene, said: "Around nine members of staff and 15 customers left the building uninjured before the Brigade arrived.
"There was smoke coming through a grill at pavement level, which has now subsided. Residents in nearby properties were advised to keep their windows and doors shut.
"Ironmonger Row and Norman Street were closed in both directions and we encouraged people to avoid the area.
"Firefighters were on scene throughout the afternoon."
The Brigade was called at 0941 and the fire was under control by 1203. Fire crews from Shoreditch, Islington, Euston, Whitechapel, Dowgate and Bethnal Green fire stations attended the scene.
The Brigade's Fire Investigators believe the fire was caused by an electrical fault in the heating system in the sauna room.