An 18-year-old is being treated in hospital with a serious head injury after an attack in Glengormley, County Antrim.
The assault happened as he was walking with two females in the Farmley Road area just before 8 pm on Saturday.
The PSNI said the teenager was punched and kicked in the face by a man who was with another female at the time.
The attacker then made off towards the Tramways area.
On Monday, police said an 18-year-old male has been arrested for causing grievous bodily harm with intent.
He is currently in custody being questioned about the attack.
It is understood the victim of the attack is a vulnerable adult with learning difficulties.
Detectives continue to appeal for anyone with information about the incident to contact them on the 101 number