Sydney school fire could be arson: police

Police believe a "very fierce" fire which ripped through a Sydney public school could be a deliberate arson attack.

The blaze broke out on the ground floor of a Parramatta Public School building in the early hours of Saturday morning.

NSW Police, on Saturday evening, said they were treating the blaze as suspicious.

No one was inside the building when the fire took hold but about 300 residents from a nearby apartment block were evacuated.

About 75 firefighters remained at the scene until Saturday morning in a bid to stamp out the final embers, Fire and Rescue NSW Superintendent Norman Buckley told AAP.

He described the fire as "very fierce" and said it had severely damaged the building, with some roof collapse.

Investigators are working to determine the cause of the fire and have called on members of the public with information to come forward.