When the Elks arrive in town, they take over. An Elk band warms up in the lobby of the Washington Hilton hotel and wails its way down the escalator. In their bejeweled regalia, the Elks get long, strange looks from the locals but, never mind, they march, 10,000 strong, down Constitution Avenue yesterday afternoon.

It's not a quiet convention: the Elks are hard to miss. An though they are the largest black fraternal order in the world, counting 450,000 dues-paying members, their purposes are largely hidden paying members their purposes are largely hidden from outsiders.

This week during their eight-day annual convention, the Elks and their families are bulging from five major hotels. Before they leave the 23,000 convention-goers estimated add nearly $4 million to local businesses.

Who are these men and women whose homespun faces are almost lost among the purple, gold and cream fezzes and tassles and the lavish velvet and rhinestone collars, the symbols of their rank in the Elks?

They form a mural of black life, the elevator operators, the morticians, the maids, the Bahamian straw worker, as well as the doctors, judges and teachers. Inside the 1,500 lodges the secrecy and the stance of race pride and self-sufficiency gives them a status the external society has historically denied.

The black Elks were created by the exclusionary policy of the white Elks. In 1898 when blacks applied to the white Elks lodge in Cincinnati, they were refused membership. Some of the blacks who then organized a black lodge were accused of stealing the ritual, lost their jobs, and, for a long time, the whites tried to prevent the use of the ritual, name and insignia. Eventually the white Elks dropped the law suits and other strategies and the blacks adopted the name. The Improved Benevolent and Protective Order of Elks of the World. Today the blacks Elks even have a few white members.

In keeping with their code of secrecy, the Elks have assumed a quiet, even guarded, but critically supportive role in black life. They are financially solvent: their own assets total $30 million and each year they spend $30 million on their multiple conventions.

Over the years in the Elks have given away $4 million in scholarships. Much of the money has been distributed through its famous oratorical contests, and the prize enabled one winner, Martin Luther King Jr., to go to college.

At the convention the business of the Elks, both the "brothers" and the "daughters" divisions, takes place behind closed doors. Out front meetings are charaterized by a loose, revivalist spirit. And it's a tempo that comes directly from the leader for the last 18 years, the man known as the Grand Exalted Ruler, Hobson R. Reynolds. Up From a Peanut Farm

"I'm the son of a peanut farmer too," Reynolds told Jimmy Carter when he mets the President in the Oval Office this week. Reynolds, a tall, spry 70-year-old who looks like a baritone in a barbershop quartet, added quickly, "I didn't vote for you but I will support you."

A few hours later Reynolds, his roaring voice cracking from strain, told a convention meeting, "Now the President wouldn't have had me there just as Hobson Reynolds but because I represent you, you and you." A few amens were heard amidst the applause.

When he moved to Philadelphia in the early 1920s from a peanut farm in Winston, N.C., Reynolds found the North as tightly segregated as the South. He locked for an organization that wanted to make a change. "It was the church and the Elks. The Elks emphasized education, and, at the time, that was considered the best way to get our goals of equality," he said.

From the lodge and deacon politics, Reynolds went to the state legislature, serving two terms in the Pennsylvania General Assembly. He introduced the first equal rights law in the state. For the last 40 years he has moved quietly in various state and federal posts, seconding the Presidential nomination of Dwight D. Eisenhower in 1952. "But without the Elks I wouldn't have had the opportunity to make the contributions I've made," said Reynolds. He has made his living as a funeral director.

He does not seem to mind a backstage role to the groups that became synonymous with civil rights activism in the '50s and '60s. "Not hollering about ourselves was the way to get work done," he said flatly. The day Rosa Parks, the Montgomery, Ala., seamstress, refused to take a back seat on the city bus, Reynolds was at a lodge meeting in Mobile. "That night we collected $3,500 and I took it right to Martin Luther King. It was the first donation he had," said Reynolds in a rare burst of boastfulness.

He'd rather talk about his wife, Evelyn Crawford Reynolds, his Baptist Church (married to both for 50 years), praise his fellow Elks, and joke about his age. "I admit to being 21, my wife knows I act like a young man," he said at an assembly where Washington Mayor Water Washington was honored. "And if the mayor's wife wants to find out . . ."

Everyone cracks up, including the mayor, who held up his hand like a traffic top. From Yale or by Mail

"If you are born black, chances are you'll die black," said Benjamin L. Hooks, the new executive director of the NAACP, at an Elks dinner this week. "Whether you got your degree from Yale or by mail . . . there's a place for the Elks in your life."

In 1952 William Lackey finished high school in Statesville, N.C., and went to work as a chauffeur for a gospel group, the Five Blind Boys of Alabama. A few years later he found himself bogged down in the monotony of a textile plant assembly line. It was then that he joined the Elks, "I always though the Elks were beautiful and someday I wanted to have that prestige," he recalled.

It took Anna Robinson, of Gary, Ind., 11 years to shake the impression that the Elks were social and frivolous. "I learned more about their work with children, the guidance available, and I decided it was the group for me," said Robinson, a mother of six and a secretary for 27 years.

Lackey and Robinson are taking the Elks philosophy to the next generation of members. They head up the National Youth Council of the Elks, and hope to shape a more active policy on the issues of youth unemployment and delinquency.

"I'm willing to use any activity to keep the kids off the streets," explained Robinson, as she checked final arrangements for the 4,000 youth delegates here this week. The highlight of the youth participation in the Elks programs is the oratorical contest and this year's winner was a 17-year-old white woman from Phoenix, Ariz., Randi Guthrie Hedin, who spoke on "What the Constitution Means to Me."

"The important thing," she stresses with the youth, said Robinson, "is to follow what the Grand Exalted Ruler always says, when you are on your way up, let's pull someone with us." Over the Back Fence

Since the convention also functions as a vacation for many delegates, the meetings are places to review the whole year with old friends. The constant chatter annoyed Nettie B. Smith, the Grand Daughter Ruler of the Temples, to the point where she publicly chastized the women in front of the men. "It disturbs me when I see the daughters visiting. Daughters, please maintain your decorum." The halls became silent very quickly and the women folded their hands in the laps of their white dresses.

Publicly she might sound like a Victorian schoolmistress but Smith has moved ahead with the times.

In recent years the women's auxiliary has participated in International Women's Year, spoken out on issues of child abuse, rape and wife beating and raised funds for the drought stricken countries of the Shael region of Africa.

"We are trying to make our presence felt today," said Smith, a diminutive woman in her 70s who was born in Washington but now lives in New York. She gave $5,000 to the development of the Elks Shrine and recreational camp site in Winton, N.C. Reynolds donated the 100 acres for the vacation lodge, and both repeatedly remind their audiences that "not a dime of ouside money" was used for the construction.

Passed down from the oldest to the youngest daughter is the obligation to contribute. "You are helping and inspiring," said Frances Mobley, 34, a supervisor at a photography studio in Charlotte, N.C. Explained Lorraine Moultrie of Charleston, S.C., a registered nurse in her 70s. "I found a new confidence. I work at a rape crisis center and as an Elk daughter, you are satisfied not only helping others but you have found a way to diminish your own selfishness."

Despite a certain flamboyance of costume, Elks don't just stand around profiling, trying to get close to a name just to be seen. At every meeting, an over-the-back fence mood prevails. "Will the lady who has the Grand Directress' pocketbook please turn it in," was the main announcement at one session. A 40-Year Elk

A convention the size of the Elks doesn't move by itself. The comfort of the Grand Exalted Ruler, the Grand Daughter Ruler and the Grand Esteemed Leading Knight, as well as Mary H. Trull, 36, of Chicago, Ill., a hostess of a Holiday Inn, is the responsibility of James C. Watson.

"Not only has Watson, 69, been an Elk for 40 years, he is also the assistant manager of the Washington Hilton Hotel. So, everyone - the hotel personnel, the Elks from Texas in their rhinestone ten-gallon hats, and the convention ladies looking for masking tape - pop in and out of his office.

"What motivates memberships even today are the parades and the flashy uniforms," said Watson, as he double-checked the parade arrangements.

Watson, a trumpet player, joined because of the music. After studying at MIT. the New England Conservatory of Music and the U.S. Military Academy, he moved to New York City, where he worked as a clerk at Harlem's then-swanky Dunbar Apartments.

"We were quite an organization in those days. As I moved along in my careers I was glad to be part of a charitable group based in the black community," said Watson. He became the first black state trooper in New York, and during his 17 years on the D.C. police force he not only organized the orchestras but was the first black in the motorcycle traffic division. Since 1965 Watson has been a Hilton employee.

Watson says quite frankly that his Elk membership hasn't gotten him any particular job but that's not what the Elks are all about. "What has kept the Elks alive is that it not only gives you something to do, but it makes you feel you are somebody." said Watson. "People who lack the proper education, who haven't finished the third grade, or who come from a socially deprived environment can find a place where someone listens. You feel respect no matter who you are."