Police say they're not sure whether the 17-year-old was invited to the function or turned up unannounced.
Riot police descended on an oval in Merrylands in the city's west on Friday night following reports of a large brawl. An 18th birthday party had been held at a nearby hall.
Inspector Adam Phillips said on Saturday between 50 and 60 males were involved in the brawl.
A young man was arrested soon after, while officers also found a 17-year-old boy with a stab wound to his chest.
He is fighting for his life in a critical condition in Westmead Hospital.
Insp Phillips said it was not yet known whether the victim was a
welcome guest.
"We are not quite sure whether in fact the victim was a gatecrasher or a person that was invited to that party," he told reporters at the scene.
A police officer was hit on the arm with a bottle while trying to break up the fight, and was treated in hospital.
Police are appealing for anyone who may have witnessed the incident to come forward
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