HORRIBLE CONDITIONS inside detention centers don't usually arouse much public concern on the outside - until inmates resort to rough stuff. But as a report by Sharon Conway in the Maryland Weekly section of this newspaper pointed out last week, there is a legal protest under way from inside the old Prince George's County Jail - and it merits citizen attention. State inmates housed in the jail have filed suit in federal court to protest "substandard and vile" living conditions in this run-down facility. Their suit, though focused on this one institution, is part of a problem that is reaching catastrophic proportions statewide.

Last year at this time, as we noted in this space, the old jail was bursting at the seams with state and county inmates - with men sleeping on floors and tables, amidst garbage. The average population was nearly five times what the jail was originally to accommodate. Of 316 men packed in the facility, 54 were supposed to be in the state prison system, but their transfer were delayed because the state system was overcrowded.

Today, the county inmates have been shifted next door, to a new county-operated detention center. But because the state has been unable to find facilities for its booming prisoner population, it is temporarily leasing the old Prince George's facility. In the inmates' lawsuit, they are charging, among other things, that conditions are such that they are sleeping on the floor in severely overcrowded quarters, that the heating system hasn't worked properly and that there are no recreation or work programs.

State officials have recognized the problem, but their search for relief has been agonizingly unfruitful. At various times, they have tried to make use of a mothballed Navy troop carrier, a dilapidated Army stockade, government surplus trailers and an abandoned East Baltimore factory. But federal red tape and the any-place-but-here objections of residents have blocked progress.

So despite efforts by Gov. Marvin Mandel and members of the General Assembly, there is no quick-fix in sight for the detention problem. Moreover, the situation in Maryland merely points up much of what is wrong with the criminal justice system nationally. To begin with, jails aren't supposed to be the same as prisons, where convicted criminals serve their sentences. Yet because of overcrowding, convicted murderers, armed robbers and others have been winding up in the same dilapidated warrens with people who can't make $100 bail on non-support charges or other lesser offenses of which they're presumed to be innocent until proven guilty. At the same time, some violent offenders are being left free for various reasons - including the reluctance of some judges to contribute to crowded conditions in correctional facilities.

At least in Maryland there has been an effort to force local decisions on sites for correctional facilities. This effort needs public understanding and support, not only for considerations of humanity but because a lot of the people who are warehoused in these prisons will ultimately return to the streets more embittered and more threatening than before.