The fire started at about 11:30 p.m. on Wednesday at the former Union Wadding Mill on Goff Avenue, most of which is unoccupied, but is also home to several apartments.
Fire officials say the occupied parts of the building were evacuated as a precaution and there were no immediate reports of injuries.
The building is more than 150 years old.
Television images showed flames leaping through the roof and walls collapsing.
A general alarm fire means that all available firefighters from area communities are requested to respond.
The cause remains under investigation.
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