Demonstrators in Los Angeles again demand that Police Chief Daryl Gates resign, A16.In the tough-cop movies, being a "Dirty Harry" means routinely and eagerly using more violence than the situation demands. In the everyday world of America's real police, the slam-bang approach to law enforcement is called "breaking bad" - and it may not be routine, but it is happening all across the nation, the experts say.

Police misconduct - especially in its uglier form, police brutality - is a social menace, analyst after analyst argues. The problem is studied dispassionately, talked about casually, worried over anxiously, condemned angrily - and it goes on and on, never stopping, possibly not stoppable, according to its critics.From time to time, police brutality returns to the surface of America's awareness, and remedies for it are pondered all over again: better ways to count the incidents and keep track of police reaction, better complaint procedures, better training for police, more civil rights cases, more damage lawsuits, and more willingness to admit there is a problem.

The problem has come rushing back to public consciousness this month, in the form of a two-minute videotape of police beating a man in Los Angeles - a gruesome scene that President Bush said "made me sick" after he had watched one of its many playbacks on television news shows.

"On the most basic level," says John Crew, who directs the Police Practice Project for the Northern California American Civil Liberties Union, "the impact of (the) Los Angeles (videotape) has been a much broader recognition of the horrible reality that police brutality does exist . . . It shakes people out of their denial that this can occur here."

His colleague from the ACLU's Southern California branch, legal director Paul L. Hoffman, says the incident "has brought desperately needed attention to the problem" in that part of the state. In Los Angeles, he contended, police brutality "has become part of the fabric of our daily lives."

It is a widespread phenomenon, some say. "This is not a Los Angeles problem," says Hubert Williams, a 25-year veteran of police work who now is president of the Police Foundation, a research organization here. "This problem exists in cities across the country. It is like the silt in the river: It is always there at the bottom of the river - not a problem unless it gets stirred up."

To Williams, and to others, it is a problem that often returns to being invisible - something, like child abuse, that people find a need to deny.

"There is a high degree of deniability to this," Williams said. "If there is no videotape (after someone complains of brutality), who are you going to believe: the police or the criminal?"

Before the nation can get a grasp on the the problem, however, it must find a way to start counting the incidents, many analysts suggest. Its dimension, in numbers, is simply not known.

Says Department of Justice spokeswoman Amy Casner: "We have some raw data, but we have to try to make some sense of it. We have to make sure our numbers are solid and accurate. I don't have any state-by-state breakdown." Says Williams: "There is no uniform data registry that allows for comparative analysis; (police) departments keep it in their own way. It's like comparing apples and oranges."

After the Los Angeles incident, the ACLU national office conducted a quick survey of 51 ACLU offices around the country, and found that as a group they receive an average of two to three complaints a day. The range was from two to three a week to 75 a week. The 75-a-week rate was reported by the Los Angeles office.

But the lack of a reliable nationwide count apparently is not the only reason for the suggested invisibility of the problem. Another major factor, it is commonly suggested, is what is called "a code of silence" that operates in many police departments, hushing up incidents of brutality.

"The video in L.A.," according to the ACLU's Crew, "provided one of the few actual cases where you can see the code of silence in action: The other officers were standing around, doing nothing. The officers doing the beating had absolute confidence that no fellow officer was going to stop them."

Adds the ACLU's Hoffman: "These officers knew that, in the ordinary course of events, they would not have to answer for this conduct."