The hostage-takers were reported to have been holding between four and six people at the church in Saint-Etienne-du-Rouvray.
A priest and two sisters, as well as churchgoers, were among those held, local media said.
France 3 television said a number of shots had been heard. Police and the emergency services are at the scene.
French TV said Interior Minister Bernard Cazeneuve was on his way to the town to be briefed by police.
