Four fire engines and 21 firefighters and officers were called to a fire at a police station in High Road, Woodford Green.
More than half of the ducting from the ground to the third floor was damaged by fire.
Around 40 people left the premises before the arrival of the Brigade. There were no reports of any injuries.
The Brigade was called at 1201 and the fire was under control at 1306. Fire crews from Woodford, Walthamstow and Chingford fire stations attended the scene.
The cause of the fire is under investigation. The Metropolitan Police Service is not treating the fire as suspicious.