Mayor Marion Barry changed his clothes at least three times yesterday. The last change, in his suite at the Washington Hilton where he and Mrs. Barry lived for the last few days, was from the conservative bluish-gray pinstripe into an even more conservative black tie.

"My advisers told me I should put on the black tie," he said, shrugging. Told that it would be projecting an establishment image, he smiled and said, "My head's all right."

City councilman David Clarke of Ward 1 walked into a reception preceding the inaugural ball last night and looked dismayed when he saw Barry in a business suit. "I was told he was going to wear a tux," he said. "That's why I wore mine." Black tie was optional at the k-per-person ball, and only a minority of the expected 5,000 guests chose that option. Also optional was a choice of music at what was being called the "Inaugural Party and Disco."

In the Washington Hilton Grand Ballroom, Peaches and Herb sang soul duets, while in the exhibition area disc jockey Donie Simpson spun records for the more athletic-minded supporters of the new mayor. Barry said that he was beginning to get used to being called "mayor."

"It's coming," he said, indicating some kind of change from the last inaugural ball four years ago, when he had told the press: "I'm struggling. Black people are always struggling."

He said he was "very pleased" at the way his inauguratiion had gone earlier in the day, "including the degermination of the kids to march in the rain."

The man responsible for making the inaugural run smoothly, Ted Hagans Jr., said all the arrangements had been made in the last three weeks: 'We made some mistakes -- people whe should have received invitations and didn't -- but I guess that happens. Otherwise, it's going pretty well. Even when the weather went bad and we had to move it in from outside it worked all right. We had a committee of 300 and we worked them very hard."

He said it was "very flattering" of Sen. Edward Dkennedy to say that he might want his services later, "but I'll be glad when it's all over. I'm a developer -- I'm building Fort Lincoln -- and I will be happy to get back to it."

Guy Draper, who made arrangements for the party along with cochairman Stephanie Greene, said that they also had had three weeks to prepare and "I'll never tackle anything like this again unless I have 10."

Many at the ball had known and supported Barry from the early days, even before he got into elective politics. One of them, Hank Wilson, who did public relations for Pride Inc. under Barry, paused for a moment to reflect on what had happened to his old friend. "Marion's always growing -- he's always changing, but it's for the better. Now he's dealing with more segments of the population than just the hard-core."

Paul Coh, manager of Peaches and Herb, has also known Barry for a long time and said, "Marion's public image has not really been what Marion is. Back in his Pride days, he had no antishite feeling, but he's been portrayed that way. He's going to be an excellent mayor, and I'm looking forward to it so much that I'm going to move back into the District."

His client Linda Green (better known as "Peaches") had a long, animated talk with the mayor while cameras snapped at an unusually photogenic subject. Asked later what they talked about, she said, "He was trying to get me to move into the District from Lanham, where I live now. He said it's going to be the only place to live and he would put me in touch with a realtor"

Her partner, Herb Fame, who was a D.C police officer for eight years before getting back into music, posed for pictures with Chief of Police Burtell Jefferson and engaged in a little bit of horseplay with his former boss.

"He fired me," he said, mock-solemnly pointing an accusing finger at his former employer. "I told him he could make thousands and thousands of dollars" the chief replied. "But I expect to see him applying for his job again, some day."

In the Grand Ballroom, which was studded with 28 cash bars and had several buffet tables with cheese and raw vegetables, the centerpiece was an enormous ice sculpture, including two huge eagles weighing 150 pounds each. "It took several men to put those in place," said one observer who had arrived early for the party. "They're supposed to last for 1 hour, but they won't be 150 pounds for very long."

Barry arrived in the ballroom somewhat later than scheduled, and he apologized both for his lateness and for his formal wear, saying, "I'm sorry we're late. Effi and I couldn't find my dashiki."

When speechmaking time arrived, around 10:30, the eagles had in fact lost some weight; but they looked somewhat more elegant in their new, slender form with more gently rounded edges than those they had at the beginning of the evening. Also slender and elegant was Mayor Barry, who assured a shouting, stamping crowd, despite the black tie, that "I will never forget where I came from. I will never forget the people who made me what I am and who I am -- and that's you, the people of the District of Columbia. I am going to be sensitive; I am going to be responsive; I am going to be open."

Explaining wryly that "the major cannot control the weather," and "we had some weather trouble in Chicago earlier today," Barry introduced Jesse Jackson, who had originally been scheduled to speak at the inauguration but was snowed in at the airport. Jackson's brief but fiery speech sounded more like campaign oratory than a celebration of victory, but it would serve well enough for either purpose.

"Marion Barry deserves to be the mayor of this city," Jackson said. "He is flesh of our flesh and blood of our blood. He is a man of the streets." He reminded the partygoers of the many people who supported Barry and who augural party, and of those who are "sitting in their homes and waiting for the bulldozers to come."

"We cannot put dope in our veins," he said. "We must put hope in our brains."

"All that Marion Barry represents to us is a chance to work without somebody putting a stumbling block in our way because of our race," Jackson said.He concluded with a stirring plea for an escalation of the mayor's responsibilities: "Marion is not only a mayor of a city; Marion is a mayor of a state. We must get statehood for Washington, D.C."

This "day of jubilee and jubilation," Barry said, was a good occasion to look at some of the problems he hopes to attack. He mentioned the infant mortality rate, teen-age pregnancies, housing and health facilities.

"We can rock," he said, "don't you worry about that. We can do that and we can get to work on Monday, too,"

"For women in this audience," he added, "this is the year of liberation."

He said that women would get a fair place and good exposure in his administration, and as long as they are "competent and compassionate," he added, "we don't care what kind of clothes they wear or what kind of lipstick they use."

After the speeches ended, Barry and his wife went down to the dance floor and did a ceremonial rock with a group of admirers standing in a circle around them. The admirers were held back by security men whose operations seemed to be patterned on those of the Secret Service -- complete with gold lapel pins and a radio receiver like a hearing aid in one ear.The security, however, was somewhat less formal than President Carter's usual encourage. After escorting the mayor across a crowded lobby to the elevator, the security men let him go up in the elevator to his suite unescorted.

A few people from the White House lower echelons were seen among the faces in the crowd, but nobody from the Carter administration was seated in the section reserved for dignitaries.

Florence Tate, the mayor's press secretary, said the White House had "indicated that they were going to send three people, but they didn't say which three. We didn't issue any special invitation except to the President."

"This is celebrity row," she said, indicating a section of tables near the stage. "They'd be here if they came."

One member of the administration, John Hechinger, who represents President Carter at the United Nations, said he was not representing him at the party. "I guess I'm representing the U.N.," he said.

Jazz musician Hank Mobley, whose friendship with Barry dates back 20 years, flew in from Chicago on a charter plane to attend the inaugural because he could not get in by a regular airline. Asked if Barry has changed, he said, "If anything, he is more militant than ever. He is militant about women's rights and he is militant about human rights, but he is the same person I knew 20 years ago."

After the members of the City Council had been introduced, Channel 7's David Schoumacher, who had been acting as master of ceremonies with three other television anchorpeople, turned to the audience.

"There are so many important people in the audience," he said, "I think you should raise your hands."

Five thousand hands went up.

What might have become a bomb scare materialized during the party when an unattached attache case was found near the bandstand. "We found the rightful owner within 20 minutes," a police spokesman said.