Up to 50 people were involved in an on-field brawl at an under-15s rugby league match in Perth, witnesses say.

The incident involving at least 20 players and up to 20 spectators is reported to have occurred during a match played on Sunday in the southern Perth suburb of Karawara.

A 15-year-old boy, who did not want to be named, told Fairfax media that there were "40 to 50 people" on the field at the height of the brawl.

"There were all the players, and 10 to 20 parents and coaches on the field," he said.

"Probably all of them were fighting, in the action, throwing punches, or on the sideline trying to get in."

Police said the disturbance had ended by the time they arrived and there were no arrests.

They said they could not confirm reports that the incident had been triggered by a spectator running onto the field and hitting one of the players.

A South Perth Rugby League Club official said the pitch invasion followed an injury to 14-year-old player who was taken to hospital after being heavily tackled.

WA Rugby League general manager Bill Nosworthy refused to comment on the brawl, saying he was awaiting information.

"This incident that has happened, we can only work through it as best we can," Mr Nosworthy said.

"When we get all the facts and figures, we can then decide on a course of action."

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