Four teenagers were robbed of their phones, iPad and bike in Wellingborough.
The three boys and a girl were in Croyland Park at about 7.45pm on Tuesday, listening to music on an iPad mini and using their mobile phones when they were approached by a man.
He threatened them, stole their phones, an iPod and a mountain bike.
The man is described as black, about 6ft, slim and aged about 18.
He was wearing black or dark tracksuit bottoms, a dark Adidas jacket with a lighter logo on the chest and red stripes down the arms.
He was carrying a small black bag and wearing Timberland-style boots.
After the incident police have issued the following advice: If you are out and about during the hours of darkness, walk with a group of friends if possible, keep away from dark and quiet places which are off the beaten track, wear a bag across your body or under your coat, do not carry all your money and valuables in one place and do not advertise the fact that you are carrying valuables like mobile phones, tablets or iPods.
Elsewhere in the town, two men and a woman were arrested on suspicion of vehicle crime on Wednesday.
One of the men and the woman, 22, was arrested after getting out of a stolen white Peugeot in Great Park Street.
The other man, 23, was arrested a short while later and two other vehicles were recovered.
All three remain in custody at the Criminal Justice Centre in Northampton.