A 13-year-old girl lost several teeth and was knocked unconscious after being hit in the face with a brick in west Belfast in the early hours of Saturday.
The teenager was sitting with friends in a playpark in Moyard Park shortly after midnight when they were attacked by a group of youths.
The attackers threw stones and bricks at the teenagers. The girl was taken to hospital and needed 14 stitches.
A possible sectarian motive is being investigated, police said.