Emergency services responded to reports of a brawl outside a Gladesville home about 11.30pm and found the boy unconscious with head injuries.

"On their arrival police were confronted by several dozen youths in the street," Inspector Mark Christie said on Sunday morning.

"We understand there were a number of youths who were uninvited."

Before the boy was punched and fell to the road, an altercation broke out among a group of males outside the home, police were told.

The males fled and the boy was taken to Royal North Shore Hospital, where he underwent surgery early on Sunday morning. His family were by his side.

On Monday morning he was in a serious but stable condition, according to a hospital spokesperson.

Inspector Christie said it was too early to comment on specific injuries or how the boy was hit but appealed for more information.

"We have a situation where there's a 16-year-old boy fighting for his life in hospital," he said.

"It's very important that the public helps us with this investigation."

It was unclear whether parents were home during the party. Some of the revellers appeared drunk.

Police from Ryde Local Area Command established a crime scene and collected witness statements but are yet to make any arrests.

Their investigations are ongoing.