Emergency services were called to the St Andrew Square area of Edinburgh after being alerted to the incident mid-morning.
It is understood there are no reports of any casualties.
The Scottish Fire and Rescue Service said they were called to the incident at 10.37am, with around 30 firefighters sent to the scene.
The fire was at office premises in Thistle Street Lane, a spokeswoman said.
Police said traffic restrictions were put in place at North and South St David Street.
The Scottish Ambulance Service confirmed it attended the scene and had been liaising with the other emergency services.
Crews, including a special operations team, were sent to the incident initially, but some left the scene when the nature of the incident became clear.