North Wales Police say parts of its headquarters in Colwyn Bay have suffered "substantial damage" in a fire which broke out in the roof.
One contractor working at the building, which is currently being refurbished, was treated for smoke inhalation after the fire which began at 1130 BST.
One hundred and fifty police, civilian staff and contractors were ordered to leave while the blaze was put out.
A joint investigation with the fire service is under way.
Firefighters used two aerial platforms to fight the blaze and crews from Abergele, Conwy, Llandudno and Colwyn Bay were involved.
People in the Min y Don area of Old Colwyn were urged to keep their windows closed because of the smoke.
The blaze was finally put out at 1447 BST, but two fire appliances remain at the scene.
The injured worker was involved in the refurbishment of the premises.
Contractors had been working on the roof during the morning.
Chief Constable Richard Brunstrom who was working out of the force area on Tuesday said he was pleased everyone was evacuated safely.
He said he was in close contact with Assistant Chief Constable Ian Shannon who is in charge.
Mr Brunstrom said policing "has largely been unaffected by the fire".
The force said damage to parts of the HQ is "substantial" and most of the older parts will be out of action "for a number of weeks".
The first task will be to make the roof from which smoke and flames billowed watertight.
A force spokesperson said an inspection of the roof was ongoing and the Health and Safety Executive has been informed.
Security at the building will be increased as a matter of course, he added.
Plans are also being made to relocated people working in the building which opened in 1974.
ACC Ian Shannon said: "North Wales Police continues to function and we continue to provide a full policing service to the people of north Wales."
Retired policeman David Curtis, 61, was working in the garden of St John's Church, some half a mile away, described seeing the plumes of smoke.
"We could see flames on the roof and then the wind shifted and you couldn't quite see because of the smoke.
"It was very alarming. I used to work in the police headquarters. I was concerned for the people who work there and for the building itself.
Bethan Lloyd was working in the adjacent civic offices when the fire broke out.
"I was leaving the office to go out on a call and I saw this huge, thick, black cloud of acrid smoke, " she said.
"All I could see was flames coming from the roof."