Two 16-year-old boys were threatened with violence, punched and then robbed in the street on Monday evening.
The incident took place just before 7pm on March 28 in South Park, by the bridge over the old railway.
The boys were approached by two men in their late teens or early 20s. They asked them for a cigarette and to borrow their mobile phones.
They threatened the boys with violence and one of them handed over a BlackBerry, but when the other refused he was punched in the face.
Police say the offenders ran off.
One was 5ft 10in, well built and had a scar on his top lip. He had short black scruffy hair. The other was 6ft and had short hair.
Lincolnshire Police are appealing for information. Anyone who saw the two men in the area at the time can call 0300 111 0300.