An 18-year-old man is in a critical condition after he was struck in the head by a group who tried to gatecrash an 18th birthday party in Sydney's north.

Police say a group of uninvited guests showed up outside the party on Keldie Street in Forestville just before midnight and tried to force their way inside.

A fight ensued, and the 18-year-old was struck in the head with an iron bar.

He has been taken to Royal North Shore Hospital where he is understood to be in a critical condition and underwent surgery.

Police have set up a crime scene at the house, and are speaking to witnesses.