The alarm about a larger brawl at Borgmästaregården came at 15 o'clock on Friday.
According to witness statements, it was a group of young adults, says the police's press spokesperson Patric Fors.
The injured boy is in his 15th year, according to Patric Fors. A tool has been used, but what kind he does not want to comment on.
An area was cordoned off for a technical investigation.
No suspect has been arrested.
