Mayor Marion Barry was the target of a series of hostile questions and mocking comments about his 3 1/2 years in office last night from a crowd of about 175 people at a meeting of the Georgetown Citizens Association. Many in the crowd were clearly angry at what they saw as Barry's broken 1978 campaign promise to oppose commercial development of the Georgetown waterfront.

Barry had promised the group then that he would support their efforts to have a park designed on vacant land remaining on the waterfront. But after Barry was elected, members of his administration came out in favor of development of the site.

Last night, Barry said that he still favors a park on the waterfront, but asserted that he was required by law to abide by a decision of his Department of Housing and Community Development that there was no legal barrier to keep developers who want to put offices on the site from doing so, providing such construction is compatible with existing use of adjacent property.

The first question to Barry was from Don Shannon, president of the citizens association, who asked the mayor why he "ran out on your promises to us from '78 on."

Barry had to wait for heavy applause and a scattering of comments to die down before he responded that he had not broken his word to the group, but was forced to go along with the decision made by his aides that the proposed development for the waterfront was "compatible" with development on other sections of the waterfront.

"I want to set the record very, very straight on this," Barry said. "Georgetown is in a historic district. . . . The only question in the Georgetown hearing was whether or not the use was compatible or not to fit in with the existing Georgetown use. That was the only question. And she (Carol Thompson, assistant director of the District's Department of Housing and Community Development) ruled that it was.

"Under the law, I cannot even speak with Miss Thompson, I cannot have communications with her, or have correspondence with her or talk with her about her decisions. So she made her decision. The citizens have several recourses. . . .

"Let me also say that reasonable people can differ. I stand here to tell you that I have an obligation to uphold the law, and if you are suggesting that maybe you ought to have a mayor who can arbitrarily and at will decide what laws to follow. . . if you want that from Marion Barry, I can tell you you won't get that."

Shannon, who was standing next to Barry, responded that, "Well, maybe you shouldn't have made promises."

Another question about the waterfront followed immediately. If Barry really wants a park there, what is he doing about it?

"I think its an eyesore and a disgrace to the nation's capital and Georgetown," Barry said of the present appearance of the waterfront. "And you are going to see it end soon. . . . You can already see we've moved our impoundment lot."

Barry had not finished answering the question when Shannon interjected that Barry was not precise when he said that the land where the city's impoundment lot had been located could be used for a park.

"There are nine acres left," Shannon said, "and 2 1/2 acres of it has been signed over by the Department of General Services, a department of your government, for use as a parking lot for a floating restaurant there. Now that doesn't sound like a park, does it?"

Shannon's comment drew applause.

The next question was a generalized one that suggested that the Barry administration suffered from a lack of integrity on the part of some of its officials. The questioner did not name anybody.

"I would be very pleased," Barry said, "if you would share with me. . . the whole question of who you think is incompetent or corrupt in the District government. I can go down the list. . . and line them up against any public employes in America and you'll find that we have the most accurate, the most competent department heads and deputies in our government. . . . I believe in integrity in government, I believe in working hard, and I believe in setting a tone for helping as opposed to hurting."

In addition to Barry, other mayoral candidates there included Betty Ann Kane, Patricia Roberts Harris and Charlene Drew Jarvis. They all addressed the group.