Officials said the 36-inch main broke at about 3 a.m. Sunday, knocking out service to many homes on the city's south side and causing low pressure in others.
Residents were urged not to use water if possible and motorists were told to avoid the area.
Lehigh County Authority spokeswoman Susan Sampson said code enforcement officials deemed three properties unsafe and evacuated residents as a precaution.
Sampson said the cause of the break on the pipe, which carries the bulk of water to the south-side reservoir and customers, wasn't immediately known.