A 14-year-old boy was treated for a serious facial injury following a "disturbance" involving two groups of males in a fast-food restaurant on Saturday evening.
A member of staff working in the McDonald's restaurant, Dunbar, was also injured during the incident, which happened shortly before 10pm inside the premises.
Police were called to the restaurant, which opened on Spott Road last November, amid reports that a teenage customer and a staff member had both been assaulted.
A police spokesman said: "Police in East Lothian are investigating following a disturbance involving a number of youths within a fast food restaurant in Dunbar.
"During this time a 14-year-old male sustained a facial injury and a member of staff also sustained minor injuries.
"Officers are currently following a positive line of inquiry, however, anyone with information that can assist with this investigation should come forward."
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