Wiltshire Police are trying to identify a member of the public who intervened during an incident at Lydiard Park in Swindon.
At approximately 4.20pm on April 22, a 16-year-old boy was hit over the head by another male near to the lake.
A metal object was used to hit him and this caused an injury to his head which later required stitches at the Great Western Hospital.
The two people were known to each other and police have arrested a 17-year-old local boy on suspicion of assault and possession of an offensive weapon in a public place.
He has since been released on conditional bail while further enquiries are carried out.
PC Craig Beales said: “When the incident happened, a member of the public stepped in and broke the two up so I am keen to speak to this person to obtain a statement.”
Anyone with information should call 101 and quote crime reference number 54190038299.
Alternatively, call Crimestoppers anonymously on 0800 555 111.
