Police used CS spray to break up a playground fight between parents at an infant school - in front of pupils.
Officers were present at Penryn Infant School in Cornwall after reports of a dispute between two mothers on the premises earlier in the day.
But as children waited to be collected, people related to the women began "behaving in a disorderly way".
Devon and Cornwall Police say officers were forced to use captor spray, a pepper-based CS spray, on a man who was allegedly assaulting a pupil's mother during the incident last Thursday. One man was arrested and has been released on bail, pending further inquiries.
A spokesman for the force said: "The use of CS spray is an effective tool for officers, particularly in public order and violent incidents, in order to prevent escalation of an incident, to bring it to a swift conclusion and prevent injury to victims and innocent members of the public."
Pc Jules Evans told the West Briton: "The male involved would not heed advice and became very aggressive and assaulted another parent. As a result of his behaviour, we were forced to use the spray and physically restrain him."
Headteacher Gillian Wright told the paper she had written to parents about the incident.
She said: "Children who are very young have witnessed something they never should have.
"That behaviour should never have happened. Safety and happiness are the most important things for children at the school."
Mrs Wright said those involved in the incident had been banned from both the infant school and neighbouring junior school until the end of term in July.