At the Hill Street Station of a major metropolitan police department, all hell broke loose some time ago. The trick now is trying to keep it confined to a limited area -- an area zoned for madness. The sheer kind.

In "Hill Street Blues," the most commendably unusual new series of the TV season so far, a gallery of believable, vulnerable and sublimely fallible characters struggle to cope with the common bad. NBC describes the show, which premieres tonight at 10 on Channel 4 (subsequently to be seen Saturdays at 10), as a "humorous police drama series," and though it is a little like a lot of other good shows and movies that have gone before it, it is not entirely like anything.

The gospel according to NBC President Fred Silverman says that a third-place network is more likely to innovate new program forms than a first-place one. The first-place one has so much more to lose. And so perhaps only on NBC would so substantial and adventurous a departure as "Hill Street Blues" have seen the light of prime time. And yet, because NBC currently commands the smallest weekly audience of the three networks, "Hill Street" may have a hard time attracting enough viewers to succeed.

If millions of people have tired of the trite and true on CBS and ABC, however -- and NBC says they have -- this bright, brash and brittle stunner from MTM Productions could find the audience it deserves. It could also do for, and to, police departments what "M*A*S*H" has done for, and to, the Army.

In the premiere, written by series creators Michael Kozoll and Steven Bochco, one is repeatedly caught off guard by the shifts from dark comedy to drama or even darkest drama, tragedy. This is never an easy thing to pull off, but it can be enormously satisfying. Bochco and Kozoll, and director Robert Butler, have put together a whale of a show, so that even when it doesn't quite work, it is altogether admirable.

Two essentially sane and decent people occupy the eye of the hurricane: Capt. Frank Furillo (Daniel J. Travanti), and his unlikely lover, public defender Joyce Davenport (the likably brassy Veronica Hamel). Furillo has to deal on the first show with a nearly rabid undercover cop (Bruce Weitz), who once bit a suspect's nose off, a community activist who wants the cops to "interface" with young people in the area, two youths who take hostages in a liquor store, an ex-wife brandishing a rubber child-support check, and a militaristic SWAT team captain (James B. Sikking) who warns him there are "a dangerous number of environmentally handicapped types out there."

A trusty old sergeant (Michael Conrad) has ditched his wife of 23 years and taken up with a high school senior. A tough-talking gang leader whose help is solicited for the local hostage crisis demands as payment an arsenal of armaments and a police escort for his mother so she can go to the supermarket in enemy turf. Not even Valium can help this guy; he'd have to take a pill as big as a Ritz cracker.

The hour is crammed with convincing, amusing and sometimes frightening details that build credibility and keep the program well away from becoming a "Barney Miller" copcom trifle. It isn't glib, and it isn't preachy; just when one thinks it might be too grimly zany, it delivers a knockout punch; two of the most likable characters are gunned down in an abandoned building.

These officers, played by Charles Haid and Michael Warren, proved indeed so likable with preview audiences who saw the show that the original ending, in which Haid died, was rewritten. He will now return, as Warren's partner, in future installments. As a salt-and-pepper team, they take up where other fictional cops left off, and Warren has an especially effective scene in which he diplomatically settles a tempestuous domestic quarrel involving a man, a wife, and a seductive stepdaughter.

Like anything new and different, "Hill Street Blues" may take some getting used to. The level of pandemonium may have to be modulated down to mere screaming hysteria for the program to wear well. But it is a pleasure to encounter an hour of television that seems overflowing with ideas, rather than the usual attempt to inflate one or two near-ideas into passable fluff.

If the quality of production and the level of invention can remain high, and the characters be made to expand and grow beyond the usual series limits, "Hill Street Blues" could be the show that the whole season is remembered by -- in charitable moments of nostalgia to come.