Four fire engines and around 25 firefighters were called to a fire at an office block in Bishopsgate, Spitalfields.
Half of a steel cooling tower on the roof of the building, which is under refurbishment, was damaged by fire. Around 65 people left the building before the Brigade arrived. There were no reports of any injuries.
Station Commander Emma Carr, who was at the scene said: "The alarm was raised by two police officers who saw smoke coming from the roof of the building and called the Brigade.
"Road closures were put in place whilst crews worked to bring the fire under control."
The Brigade's 999 Control Officers took 15 calls to the fire.
The Brigade was called at 0906 and the fire was under control by 1019. Fire crews from Shoreditch, Dowgate, Shadwell and Bethnal Green fire stations were at the scene.
The Brigade's Fire Investigators believe the fire was accidental and caused by hot works being undertaken within the immediate vicinity of combustible materials.