A dramatic predawn armed robbery Sunday shook the nerves of a Manteca pharmacy and drug store staffers in the 1000 block of North Main Street.Manteca Police received a call reporting an armed robbery in progress at the store at 4:41 a.m. Officers were on the scene within two minutes of the call, hoping they could detain the robbers while still in the store.The first robber described as being a black male, standing about six feet tall jumped the counter. Armed with a hand gun, he confronted a staffer demanding narcotics from the pharmacy. The employee reportedly pointed to the narcotics and fled to the rear of the store as the robbery was reported to another employee.