Two teenage boys had bottles thrown at them before one of them was held up at knifepoint on their way home from a school prom in Doncaster.
The boys were walking along Mill Street, Armthorpe, when they were targeted at 12.40am on Saturday, June 25.
South Yorkshire Police said a group of men hurled bottles at the boys before they ran off and became separated.
One of the boys was then ambushed by two men, one of whom was armed with a baseball bat and knife.
The men stole the teenager's mobile phone.
A South Yorkshire Police spokeswoman said: "Two teenage boys were walking home from their school prom along Mill Street when they were approached by a group of men unknown.
"The group are alleged to have thrown bottles at them before the two boys ran away.
"One of the boys, who is 16, separated from his friend and has then been approached by two men from the group, close to the junction with Larch Drive.
"One of the men is described as holding a baseball bat and a small pen knife.
"The two men have then threatened the victim, before stealing his mobile phone. He was not injured during the incident."
One offender was white, 18 or 19 years old, between 5ft 6ins and 5ft 8ins tall, of a medium build and had dark brown, curly hair.
He is thought to have been wearing a light grey jacket.
The other was white, 17 or 18 years old, slim and under 5ft 6ins.
He had dark, straight hair and was wearing an Adidas top with three stripes down the sleeves.
Anyone with information should call South Yorkshire Police on 101 or Crimestoppers on 0800 555111.