Dozens of people had to clear Cambridge's historic Guildhall when a fire broke out overnight.
A member of the public spotted the fire in a ground floor room of the city council offices and called emergency services at about 0100 GMT on Saturday.
A Cambridge City Council spokesman said the fire had been confined to the one office and most of the damage was from smoke and soot.
Service recovery co-ordinator Arthur Savage said about 50 people had been in the building after a party to celebrate the Chinese New Year when the fire started.
Scenes of crime
"There had been a late event in one of the public halls and people were still clearing up," he said.
A Cambridgeshire Fire Services spokesman said everyone had been safely accounted for.
Two fire engines and a turntable ladder attended the fire which was out shortly after 0200 GMT.
Scenes of crime experts and a fire investigation officer were combing the scene on Saturday.
A police spokesman said they had not established how the fire started.