The speed and deadliness of recent high-profile shootings have prompted police departments to recommend fleeing, hiding or fighting in the event of a mass attack, instead of remaining passive and waiting for help.

The shift represents a “sea change,” said Chuck Wexler, executive director of the Police Executive Research Forum, which recently held a meeting in Washington to discuss shootings like those in Newtown, Conn., and Aurora, Colo.

The traditional advice to the public has been “don’t get involved, call 911,” Mr. Wexler said, adding, “There’s a recognition in these ‘active shooter’ situations that there may be a need for citizens to act in a way that perhaps they haven’t been trained for or equipped to deal with.”

Mr. Wexler and others noted that the change echoes a transformation in police procedures that began after the shooting at Columbine High School in 1999, when some departments began telling officers who arrived first on a scene to act immediately rather than waiting for backup. Since then, the approach has become widespread, as a succession of high-profile shootings across the country has made it clear that no city or town is immune and that police agencies must be prepared to take an active approach.