A group of yobs spat at and assaulted two teenage girls before hitting a Good Samaritan over the head with a bottle.
Police have launched an appeal for witnesses after the incident at the Saffron Road recreation ground in Higham Ferrers.
Two girls were near the skate park between 6.45pm and 7.40pm on Friday, August 30, when they were assaulted by a 'large group' of young people.
They verbally abused them, threw objects at them, pushed them and spat at them.
When two adults came over to intervene, they were also attacked by members of the group with one man hit over the head with a glass bottle.
A police spokesman said: "The group of young people are described as a mix of teenage boys and girls aged 13 to 18.
"One of them drove away from the scene in a small white vehicle."
Anyone who saw the incident or has information about it should call Northamptonshire Police on 101.