A boy was left with cuts and bruises after an attack by up to six other teenagers.
The 15-year-old victim from Mayfield was surrounded by the gang in The Avenue, Mayfield, at about 6.20pm on Friday.
He was punched, kicked and slashed with a knife – but luckily the blade only cut his shirt.
His ordeal ended when he managed to run away.
PC Donna Lonsdale said: "Luckily the knife only cut the victim's shirt rather than him but this could have been even more serious than it was.
"The victim has been left with bruises, cuts and sore ribs and is very shaken.
"I need to speak to anyone who was in Mayfield on Friday evening and saw the attack or a group of teenagers walking around the area.
"This was a motiveless attack and we need to track down those responsible as soon as we can."
One of the teenagers is described as white, 16 or 17, athletic and 6ft or 6' 1". He had brown spiky hair and stubble. He was wearing a grey jumper and grey tracksuit bottoms.
Another of the teenagers was white, 5' 7" or 5' 8", about 15 and skinny. He had shaggy bleached blonde hair and was wearing a white or cream hooded top and tracksuit-style shorts that cuffed around the legs.
Anyone with information should call 101 quoting serial 388 of 14/6, email firstname.lastname@example.org or call the independent charity Crimestoppers anonymously on 0800 555111.