The man lay motionless on his back like a huge overturned beetle, his knees and forearms cocked in the air as he slept a deep inebriated sleep.

Steam from the heat vent where he lay at Constitution Avenue and Ninth Street NW shimmered around him, giving off a damp warmth in the night air. Four other bodies were slumped around the vent. Their belongings - empty wine bottles, a tattered Safeway bag, bits of string, remnants of clothing - were drawn close to them. One man raised himself on an elbow and took savage bites from a boiled potato given him by a passerby.

There was no sound except the hiss of steam.

These are the street people of Washington. There are hundreds of them, perhaps thousands. By night, they crowd onto the heating vents around government buildings or crawl into abandoned houses and garages. By day, they walk the streets and wander in the parks.

"It's a walking world," says Mitch Snyder, 35, a member of the Community for Creative Nonviolence, a radical Christian group that is organizing efforts to move the street people into the National Visitor Center at Union Station as emergency night shelter for the winter. "There's a subculture in Washington that walks. They're on a pilgrimage that never stops . . . a forced march."

The city government and several private mission houses already provide emergency shelter and food for hundreds of destitute and homeless people each night. But, according to workers for the Christian group, there is a hard core of street people who, even if such beds and food were available, would not or could not accept them.

"Many of them can't take the hassle, the controls," Snyder says. "They don't trust the system . . . They don't want to have to give a name or a Social Security number at the [government] shelters, and they can't take the religious trip at the missions."

Many hard core street people are alcoholics. Some are mentally or emotionally impaired. "The one thing they have in common," Snyder says, "is they can't play the game any more."

Hence the Community's plan to open up the visitor center for street people. Under the group's auspices, there would be a minimum of regimentation. Homeless people could feel free to come, have a hot meal, bed down on a plastic and foam rubber mat for the night and leave the next morning without signing in or being interrogated, Snyder says.

Beyond that, Snyder freely admits, CCNV's confrontational tactic of telling - rather than asking - the U.S. Interior Department that it is "opening up" the visitor center for street people at night is calculated to generate public attention and, hopefully, to force the government to find a permanent solution to the problem of the homeless.

CCNV volunteers, meantime, have been providing food and temporary shelter on a spot basis for street people in scattered parts of the city.

On a night earlier this week, Mark Lee, a bearded 22-year-old worker for the group, chugged around the city in a yellow van, stopping off at heating grates and other gathering places to distribute warm boiled potatoes wrapped in tinfoil to street people.

Some were unconscious, lying bound like mummies in tattered blankets and coats. Others looked up dully at the proffered food and accepted it, grunting thanks.

Most street people are men, some white, some black. A larger older man lying on the grate in front of D.C. police headquarters at 300 Indiana Ave. NW coughed and wretched convulsively, ignoring the food. Another man was sprawled unconscious on the vent by the Corcoran Gallery of Art on New York Avenue NW, his clothes soaked with urine.

During the long waking hours of the day, the street people walk, wander and sit. In the parks, they must sit, not lie, on the benches. Otherwise, U.S. Park Police will roust them out. Their world becomes a set of zig-zag routes between the heating grates, public bathrooms, trash bins known for their supply of edible refuse, carry-out restaurants where they can panhandle a quarter for coffee, the bus stations, libraries and a few other sanctuaries.

"They are invisible people," says Snyder. "They walk through the city all day, and nobody knows or wants to know they are there.

Snyder, a one time management consultant who joined the Community in the early 1970s, has emerged as one of its principal spokesmen in the visitor center effort.

He and another member, Ann Splaine, moved out of their communal home at 1345 Euclid St. NW two weeks ago and have been living in the streets, sleeping on the heating grates at night, talking to the derelicts and street wanderers about the visitor center project and seeking further dramatization of the plight of the homeless.

A powerful speaker with dark piercing eyes, Snyder has drawn criticism from some church and peace groups for his adventurous tactics. Often arrested, he once vaulted the White House fence during a Vietnam war protest. More recently, he fasted for more than a month in an unsuccessful effort to get a well-heeled Roman Catholic parish in Georgetown to disvert a portion of its church building improvement fund to the poor.

Describing himself as a Christian with an anarchic streak, the Brooklynborn Snyder says living in the streets has taken him "beyond the intellectual stage of simply viewing" the plight of the homeless to a direct, "almost mystical understanding" of it.

"There's an insanity, an absurdity," he said in an interview in Lafayette Park in front of the White House. "These people out here are hungry, sick, broke, degraded, intimidated, the most miserable segment of society. Yet they are ignored . . . Our social and economic systems drive people apart, instead of bringing them together. that is why I feel the need for radical change."

Living on the streets, he has learned the "blessing" of drinking cheap brandy to dull the pain of the perpetual cold, he says. He has become a connoisseur of the heating vents. Some are too hot, some are too damp, some have howling winds that make sleep impossible.

"Street people are generally quiet, gentle people," he said, although some of them will steal and assault.

"It's like a mirror of the larger society," he said. "There are those who will share a sandwich they've picked out of a trash can and those who will hurt and steal . . . You often find men sleeping together holding hands. They're not gay . . . It's a great human gesture of humanity."