Some of the most powerful men in Washington were there, lining up at the small reception table outside the Congressional Room of the Capital Hilton Hotel yesterday.

A few came with several checks in envelopes or in their hands, ready to dole out the $500-a-plate fee to have lunch with the man who 10 years earlier had called them "merchant moneylords," but who in about 10 weeks, many of them felt, would become the next mayor of the nation's capital.

City Council member Marion Barry, the Democratic nominee for mayor, was the first choice of very few of these bank presidents, utility firm heads, retail executives, accountants, realtors, restaurateurs and others. Many of them preferred, contributed to and worked for Mayor Walter E. Washington or City Council Chairman Sterling Tucker.

But Marion Barry won the Sept. 12 Democratic primary, and he had suddenly become the man of the hour, if not yet the man of the next four years. So there was hardly a major businessman in town yesterday who asked for a raincheck on this luncheon date.

"Look at all that power comin' in here," joked Joseph H. Riley, president of National Savings and Trust Co.

"I supported him all the time," said a grinning Joseph B. Danzansky, chairman of Giant Foods who actually supported Tucker.

"I gave him $400," chimed in Foster Shannon of the Shannon & Luchs real estate firm, who had contributed to all three candidates in the Democratic primary.

Leonard B. Doggett Jr., the parking lot owner and developer who raised money for Walter Washington , had sold an entire table - $5,000 - worth of tickets for yesterday's Barry luncheon.

"I thought you were supporting Walter Washington," a reporter asked Doggett.

"Who?" Doggett asked.

"Walter Washington."

"Which way is the bar?" Doggett responded.

Within two hours, Barry's campaign organization was $60,000 richer, according to luncheon organizer Ann Kinney.

Barry raised more than $250,000 during his primary election campaign, but throughout the primary, he was unable to attract large contributions from the city's major businessmen. He relied instead on donations of $250 or less while his two opponents garnered more contributions of $500, $1,000 and more.

Yesterday, however, it was a different story. "We couldn't raise $500 a person before the primary," said Barry supporter Delano Lewis, winking an eye as he stood outside the door watching the line-up of businessmen.

The power and rules of politics and the continual quest for access to the seat of government had pulled this crowd together. Many had mortgaged their hopes for an ear in city hall on other candidates who had lost. Now Barry was offering second mortgages, and it was a seller's market with an even greater potential than he had expected.

"You'd be here, too, if you were a business leader and the guy that you did not support won and you wanted him to know that you supported him," said realtor Jeff Cohen, a longtime Barry supporter. "I know I would."

Barry, who is facing Republican Arthur A. Fletcher and two others in the Nov. 7 general election, used the occasion to poke fun at his own former image as a rabble-rousing black militant who has gradually become a stylish member of the city's establishment.

Apologizing for a late start, Barry said, "I had to stop by the cleaners. I was looking for my three-piece dashiki. I couldn't find it."

During a half an hour speech to the businessmen, Barry acted as if winning the November election were only a formality. He spoke constantly of what "the Barry administration will" do. He called on the businessmen to become more actively involved in other city activities.

Barry's was not a rah-rah speech. It was interrupted twice by applause. Instead, the 42-year-old Democratic nominee outlined his goals for the city in a wide variety of areas, including housing, education, job training and economic development. Most of the goals were identical to those he had spelled out during numerous campaign appearances.

He promised a change in the way city government would be run, making it more efficient and more sensitive to people, and he pledged a more aggressive style of leadership. He told the businessmen, many of whom at times had felt that Mayor Washington deserted them in the past, that he would always seek their opinion on taxes and other legislation affecting them.

In areas of particular interest to the business community, Barry said he would strengthen the city's new Department of Business and Economic Development, streamline procedures for obtaining necessary business licenses and permits and increase the city's ability to install public improvements such as roads and lights needed by new businesses.

He also pledged to help revitalize the city's downtown as "a living downtown, as opposed to a 9-5 downtown" and to work for completion of a downtown convention center.

Many businessmen appeared to leave with a new interest in Barry. "I really don't know Marion," said former Tucker supporter Harold Bobys, managing partner of Alexander Grant & Co., an accounting firm. "But I really think there's got to be a lot him than we gave him credit for."

Bobys said in the past he had been hesitant to support Barry because of Barry's "track record."

The guest list for the fund raiser read like a who's who of the city's business community: Edwin K. Hoffman, president of Woodward Lothrop; Thomas J. Owen of Perpetual Federal Savings & Loan Association; W. Reid Thompson of Pepco and R. Robert Linowes, president of the Metropolitan Washington Board of Trade, who had helped to organize the luncheon.

There was also Ulysses (Blacke) Auger, the restaurateur; Gilbert N. Violante of the Washington Parking Association; Israel Cohen, president of Giant Foods; K. Donald Menefee of Madison National Bank and builders Morton Funger and Nathan Lanow.

Also realtors Flaxie Pinkett and H. R. Crawford, former Police Chief Maurice J. Cullinane (now a registered lobbyist for the D.C. Bankers Association), lawyer Clinton Chapman and builder Oliver T. Carr Jr.

The large showing seemed to place a damper on the hopes of Republican Fletcher to draw large support from the business community. Fletcher's reports filed with the city elections board yesterday showed he had raised only $28,446 for his campaign, while Barry has raised $296,059.

At yesterday's luncheon alone, Barry raised more than twice as much money as Fletcher has raised all year. Since Sept. 7, when reports were last failed, Fletcher has received $6,916, while during the same period Barry has received $31,696.

"Fletcher's a very impressive guy, very impressive," said Board of Trade president Linowes. "But Marion is a known quantity." CAPTION: Picture, Joseph Danzansky and Marion Barry greet each other at $60,000 lunch. By Charles Del Vecchio - The Washington Post