The incident happened on Thursday in the town of Beauvais, north of Paris.
Footage shows a man apparently speaking to one officer at the side of a road in the Saint-Jean area of the town.
Another office nearby, holding a dog on a leash, suddenly runs towards him for no apparent reason and pushes him over.
The first officer walks away while the two men then roll around on the ground until they are separated by other policemen.
Nothing is yet known about why the police officer reacted the way he did.
The mayor of Beauvais, Caroline Cayeux, said the incident was serious and would be investigated.
According to local newspaper L'Observateur de Beauvais she met the man's family on Thursday afternoon.
"The mayor has demanded an inquiry to shed some light on this," said a statement.
See the incident for yourself. The attack happens at around 27 seconds.