A mass brawl has stained the Manly under-17s grand final game with police inquiring about reports a player was king-hit and then kicked in the head.

Players rushed from the benches to join in after violence erupted during the match between between Avalon and Beacon Hill on Brookvale Oval on Sunday.

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Video footage shows players launching themselves into the fight with several clearly seen kicking and punching into the mass of bodies.

The brawl started after an Avalon player was tackled heavily and then reportedly king-hit, falling unconscious to the ground.

"I heard that he was also kicked in the head while he was on the ground," Parent Nici Andronicus said. "For the sport to be brought into such a violent episode of utter destruction is just horrific."

View photos Parent Nici Andronicus said she found the incident to be horrific. More

Police said in a statement they had been made aware of an incident and were investigating.

There are also claims of racial abuse towards Beacon Hill players.

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Media personality and former NRL star Matt Johns was present at the game and watched his son Cooper kick a field goal that brought Avalon their 21-20 win.

Johns told Triple M that the young player who was hit lay on the ground for "10-15 minutes with the ambulance called”.

“There is every chance this won’t be a junior league matter, it will be a police matter," Johns said. "It will be an ongoing story.”

NRL CEO Todd Greenberg said in a press conference the incident was "disappointing".

"We work pretty hard to make sure junior rugby league participation rates continue to be invested in but those sorts of things take us a step backwards."