The 1990 U.S. Census began Tuesday night on a somber note, as census takers set out to measure the size of one of a successful nation's most conspicuous failures--its population of homeless citizens.

With a budget of $2.7 million and a staff approaching 15,000, the U.S. Census Bureau attempted to count on one night a population that could number in the millions. It is the first time the census has counted such people separate from the rest of the population, most of whom are to receive census forms in the mail later this week.

Census officials concede their count of homeless people is not expected to be complete.

At a Washington press conference Tuesday, Census Bureau director Barbara Bryant said that while the homeless count would not be exhaustive, "these numbers will be believable. Is it hundreds of thousands? Is it millions? We're going to show that kind of scope."

In Los Angeles, John Reeder, head of the Census Bureau's California operations, said the count would be a "conservative" one.

Speaking at a Tuesday press conference, Reeder said that the bureau was prepared to continue counting homeless every night this week if it became apparent Tuesday that there were more homeless people than the census takers could handle.

Reeder made it clear census takers would not take certain risks.

"We are not going on rooftops, looking in abandoned cars or peering into dumpsters," he said.

Nonetheless, the novel exercise was shaping up as an all-night, national spectacle.

In San Francisco's Golden Gate Park, the census takers, wary of ambush, planned to keep to the park's lighted perimeters, calling out for any homeless persons hiding in the bushes to come forward and be counted. In the nation's capital, wary homeless advocates were attempting to thwart the census, refusing to allow counters inside a shelter where 1,000 homeless were said to have congregated. And in Los Angeles, buses were being hastily arranged to haul homeless from the rougher streets of South-Central to armories and emergency shelters for counting.

Reeder said that six units of U.S. Naval Reserves would be patrolling the streets of Los Angeles to help ensure the safety of census takers.

"Obviously, we are concerned about security. We cannot take police because it would breach confidentiality," he said. But he added that there would be an increased police presence on the streets of the city while the homeless census was in progress.