A 16-year-old boy was left unconscious after being attack in a Chorley park.
The boy was walking through Cuerden Valley Park on Sunday when he was hit on the head from behind.
He fell to the floor, was knocked unconscious, and was found lying on the path by another walker.
It happened at 11.30am near to the Stag Heads Car Park off Wigan Road.
The teen was taken to Royal Preston Hospital where he was treated for a cut lip, bump to head and tender jaw.
PC Paula Farrow said: “The motive for the attack is unknown at this stage but this is a serious and completely unprovoked assault and we need the public’s help to find the person responsible.
“This area is popular with dog walkers, particularly on a Sunday so I would appeal to anybody that thinks they may have witnessed this assault or anyone that saw any suspicious activity in and around that area and that time to contact me at Lancashire Police on 101.”
People with information can also contact the independent charity Crimestoppers anonymously on 0800 555 111 or on-line at Crimestoppers-uk.org.