Police officers responding to an active shooter are trained to proceed immediately to the area in which shots were last heard; their purpose is to stop the shooting as quickly as possible. The first responding officers will normally be in teams of four (4); they may be dressed in regular patrol uniforms, or they may be wearing external bulletproof vests, Kevlar helmets, and other tactical equipment.
The officers may be armed with rifles, shotguns, or handguns, and might also be using pepper spray or tear gas to control the situation. Regardless of how they appear, remain calm, do as the officers tell you, and do not be afraid of them. Put down any bags or packages you may be carrying and keep your hands visible at all times; if you know where the shooter is, tell the officers. The first officers to arrive will not stop to aid injured people; rescue teams composed of other officers and emergency medical personnel will follow the first officers into secured areas to treat and remove injured persons.
Keep in mind that even once you have escaped to a safer location, the entire area is still a crime scene; police will usually not let anyone leave until the situation is fully under control and all witnesses have been identified and questioned. Until you are released, remain at whatever assembly point authorities designate.