Five firefighters are currently working to bring the blaze under control at Nottinghamshire County Council offices. Smoke comes from a first floor window.
A spokeswoman for the Nottinghamshire Fire and Rescue Service said: “The time of the call was at 11:45am.
“We have 12 pumps that are either in place or on the way.”
Fire trucks from London Road, West Bridgford, Stockhill, Edwinstowe, Mansfield, Carlton, Stapleford and Bingham stations were called in.
The fire service expected to be on the scene for “considerably long”, as the first floor of the building was “extremely smoke-filled”.
It is not clear at this time what caused the fire.
Council leader Ben Bradley MP confirmed that everyone in the building was safe at the time of the fire.
The historic headquarters are on Loughborough Road on the banks of the River Trent.
They were built in the 1930s and opened in 1946, making them the oldest and largest building owned by the municipality.
The site houses the council chamber for meetings in addition to services and support offices.