A Sydney teenager is fighting for life in hospital after suffering a head injury during an assault at a gate-crashed Halloween party.

The 16-year-old was punched outside a home on Ganora Street, Gladesville, about 11.30pm on Saturday, causing him to fall onto the roadway and hit his head.

Several neighbours called police when the brawl broke out and officers were forced to deal with dozens of partygoers in the aftermath.

Detective Inspector Mark Christie says police don’t believe the boy was a resident of the house and they’re still trying to work out whether he was an invited guest.

“We understand there were a number of youths who were uninvited who were at the scene,” he said.

A number of revellers fled and police are eager to speak with any witnesses who haven’t yet come forward.

Detective Inspector Christie said it was unclear whether parents were at home at the time and police are investigating whether the boy was king hit.

He was taken to Royal North Shore Hospital in a critical condition and underwent surgery overnight, but his condition had not improved on Sunday morning.

Detective Inspector Christie said the teenager’s family was with him.

“They’re very upset,” he said.

Footage of the scene shows officers grappling with youths, some of whom would not leave.

Detective Inspector Christie pleaded with anyone throwing a house party to consider the consequences of supplying alcohol to minors.

“Not only is it illegal but it can be very dangerous,” he said.

“If you’re going to have a party, make sure it’s controlled, make sure there is adequate supervision.”

– AAP