A pair of teenagers were approached by older boys in Northampton park and one was attacked.
The crime happened in Abington Park, Northampton, on Friday, June 24, between 3.40pm and 4pm.
The two 14-year-old boys were approached by two older boys near the bowling greens.
A police spokeswoman said: “The older boys asked the victims if they had any money and then searched one of them. When the other victim refused to be searched, one of the offenders punched and kicked at him.
“Both victims ran away towards Ardington Road and were chased for a short time by the offenders.”
One of the robbers is described as black, about 16-years-old with a medium build, black hairs on his chin and a high, black afro with shaved sides. He wore a blue NorthFace coat and a red hoody.
The other was about 5ft 7ins with a slim build. He wore a navy balaclava with a white pattern on it, a blue hooded top with the hood up, black jeans or tracksuit bottoms and blue and white Adidas Flux trainers.
Officers would like to hear from anybody who was in the park on the date mentioned above and saw two people matching these descriptions, or who witnessed any similar incidents.
Witnesses, or anyone with information, are asked to call Northamptonshire Police on 101. Alternatively, they can call the independent charity Crimestoppers anonymously on 0800 555 111.