The incident has provoked a wave of indignation from locals in support of the mayor, Maurice Boisart, as well as from other mayors who are worried about the impact of a judgment against their colleague.
Local newspaper La Voix du Nord reported that the incident happened after a 17-year-old boy climbed over a fence to get his ball in the small town of 2,500 inhabitants close to the Belgian border.
The mayor told him off and was allegedly insulted and threatened by the boy, leading to the slap. The father of the boy lodged a complaint about the incident.
"Good luck", "we would have done the same thing" and "what you did is completely excusable; the boy deserved it" wrote locals in a book containing over 38 pages of messages of support in the town hall.
Angry council employees have pledged to go to the tribunal to support the mayor on Tuesday afternoon.
Mayor Broisart said he has also received messages of support from other elected mayors in the region.
"Between 70 and 80 have written to me," he told the newspaper. "Mayors, councillors, senators."
Many of the officials have said they will be at the tribunal to support their colleague.
"I've already been the victim of an attack," said one local mayor, Philippe Léty, from Solre-le Château. "Sometimes things go too far and it's hard to contain yourself."
"Rudeness has always been around," he said, but "what's new is the non-respect."
Other mayors felt that things had gone too far in the case.
"If the judge says he shouldn't clout him, that's OK, but things shouldn't have to go this far," said the mayor of Bavay.
Mayor Boisart himself is anxious about the hearing. "I wonder what's in store for me," he said.