The two men took hostages at a church in Saint-Etienne-du-Rouvray, near Rouen in the département of Seine-Maritime (see map below).
Reports say the men entered the church during mass via a rear door. It is unclear what their motives are.
Specialist elite police units are on the scene and a security cordon was set up around the church.
It is believed a priest, two nuns and two worshippers were among the hostages,while another nun was able to escape and raise the alarm.