Crews from around the county arrived to fight the blaze, which has now been put out
The County Hall in Lewes - the headquarters of East Sussex County Council - has been hit by a fire.
Six fire engines were called in to fight the blaze on St Anne's Crescent, which was reported at about 06:45 GMT.
The fire has now been extinguished but crews remain at the scene damping down, East Sussex Fire and Rescue said.
No injuries have been reported and parts of the building are now reopened and back in operation.
The cause of the fire is under investigation.
The fire was on the second floor of the building
East Sussex County Council's Chief Executive, Becky Shaw, said: "We are extremely grateful to the [fire service] for their prompt attendance and pleased there were no injuries.
"The fire has affected only one section of the building and parts of County Hall are open for business as usual.
"We do not expect any serious disruption to our services."
