A policewoman was reportedly injured with a knife near the French city of Nantes.
The perpetrator fled.
The Gendarmerie Nationale warned the population on Friday via Twitter to avoid the area in the city of La Chapelle-sur-Erdre.
The incident is said to have occurred there in the morning, as reported by the newspaper "Ouest France".
Accordingly, a man entered the police station and attacked the woman with a knife.
He then fled.
A large contingent was looking for the attacker at noon.
The policewoman is said to have been badly injured in the thigh and was taken to hospital.
The background to the incident was initially unclear.
Only a few weeks ago a policewoman was killed with a knife in a community near Paris.
According to the investigators, the perpetrator acted with an Islamist motivation.
Further attacks on the police had recently triggered a major debate in France about the safety of the emergency services.
The death of a police officer while working against drug-related crimes in Avignon had also caused an uproar.