WHO ARE ALL these people huddled on the heating grates outside government buildings, department stores and around the nation's monuments? Why have they kept upsetting our Christmas cheer as we hurried home from office parties and present-buying? Isn't there some place for them to go where, at least, we won't have to think about them all the time?

These are the dregs of the nation's homeless -- the winos, the beggars and the bag ladies. They're more obvious these days because they have fewer places to go. They used to spend their nights in jails or single room occupancy hotels. Many once resided in public mental institutions. But urban renewal has displaced the flophouses and missions in many cities. And civil rights activists persuaded courts that it wasn't right to lock people up just for being drunk -- and anyway the jails are full with other, more serious offenders.

Perhaps a quarter of the street people are refugees from mental institutions. They were dispossessed by the enthusiasm for "deinstitutionalization" that swept the country a few years ago. Well- meaning liberals argued that it was wrong to keep people locked up now that new drug therapies make it possible for many mentally ill people to function fairly normally. That argument also appealed to the fiscal conservatives, who saw great savings from closing the institutions that once housed them. The solution worked well for those patients who could return to jobs or caring families. But the halfway houses and job opportunities that were supposed to sustain most of those ejected never materialized in even near sufficient number.

Neither did the rehabilitation centers that were supposed to handle the alcoholics. Nor the substitutes for the cheap housing and shelters displaced by urban renewal. Now all their former occupants are crowding the soup lines, missions and hastily constructed public shelters in cities from coast to coast. Together with the long-term unemployed, many of these people are sleeping on grates and in makeshift cardboard shelters or sitting up overnight in church pews and public restrooms. Occasionally some will threaten suicide or other violent acts in hopes of being locked up in a warm safe institution. And occasionally some of them will freeze to death.

It's not easy to help some of these people. Many won't go to shelters because they fear -- often with good reason -- that they will be brutalized by other occupants or even by shelter guards. Many are terrified of human contact, lost in the horrifying world of the insane. But it doesn't require a full flight into madness or alcoholism to earn you a spot on the grates. All it takes is a broken family, lost savings, a disability that won't meet the government's toughened standards for aid or a Social Security or welfare check that won't pay the rent.

Now and then you will see a street person -- a bag lady, perhaps, with her head held high and an air of faded gentility about her shabby clothes -- who will remind you that, even in this wealthy nation, there is no sure guarantee against having to sleep under bridges and beg in the streets.