A teenager is in a critical condition and a police officer is being treated for an injury following a brawl outside a party in Sydney’s west.

Emergency services and the dog squad were called to the Merrylands home about 11pm on Friday where they found the brawl in progress and the 17-year-old youth with a stab wound to his chest.

Police moved in to break up the fight, during which an officer was hit on the arm with a bottle.

Both the youth and the officer have been taken to Westmead Hospital for treatment.

Investigators have been told a party was being held at a hall on a nearby oval.

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