UPDATE: Mother says teenager stabbed at party was a gatecrasher

A mother who hosted a birthday party at which a 17-year-old was stabbed says the victim, now fighting for his life in hospital, was a gatecrasher.

Riot police descended on an oval in Merrylands in the city's west last night following reports of a large brawl near the hall where the 18th birthday party had been held.

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.

The mother who hosted the party, who cannot be identified, said 80 people had originally been invited to the party, but unwanted guests had showed up.

She said the gatecrashers were armed with knives, and their presence led to a brawl in a carpark during which the teen was stabbed.

She also said the incident was a warning for other parents thinking of hosting a party.

A police officer was hit on the arm with a bottle while trying to break up the fight, and was treated in hospital.

Inspector Adam Phillips said between 50 and 60 males were involved in the melee.

Police are appealing for anyone who may have witnessed the incident to come forward.