A principal assistant and all seven interns in the public affairs office of the National Endowment for the Humanities resigned yesterday, charging they are victims of reprisals over news leaks about the planning of a surprise party for Joseph E. Duffey, the agency chairman.

Duffey, following newspaper reports that a poolside party honoring his 46th birthday had been arranged on government times, ordered an in-house investigation by his deputy for management, John Whitelaw.

Whitelaw later reported that interns in the office of Kay Elliott, the public affairs director, had been improperly detailed to work on the affair. He said the agency would be reimbursed by Duffey and his wife, presidential assistant Anne Wexler, who had asked Elliott's help in arranging the party.

The interns, meeting with reporters yesterday afternoon in a 15th Street restaurant, acknowledged that they had leaked the party story.

They charged that as a result of the newspaper stories, Elliott had "abruptly curtailed" their duties at the endowment, and prompted them to request a 90-minute meeting on the topic this week. At the meeting, according to three of the interns, Elliott stated that "you caused me all the problems."

"Our budget is in a state of emergency in the senate," they quote Elliott as telling them. "We're being audited and we are no longer allowed to give any more luncheons. You blew it for me."

"Basically," one of them continued, "she blamed it all on us."

Asked for comment last night, Duffey said: "No such statement was made on my behalf or on the agency's behalf. I can understand that it may have been made under the press of the moment but it's just incorrect in every respect."

Elliott was unavailable for comment yesterday.

The interns were joined at the restaurant meeting by David Braaten, a former reporter for the Washington Star, who came to Elliott's office as a consultant in March.

Braaten submitted an emotional letter of resignation to Duffey yesterday naming Elliott as "the one person guilty of any wrongdoing" in "the whole incredible megillah over your birthday party."

Braaten charged in the letter that "the endowment brass has, in a manner frighteningly reminiscent of Watergate, attempted to deal with the situation by harrassing everyone suspected of talking to the press (i.e. of telling the truth) and backing up (Elliott)."

When Duffey was first informed of the planning of his own surprise party - by reporters who reached him at home in the late hours of June 29 - he had characterized Elliott's use of the interns as a "colossal mistake." Then and subsequently, however, he had high praise for her abilities as his principal aide for public relations, and repeated it yesterday.

Later Wednesday, Whitelaw, the deputy director who investigated the birthday party incident, had a meeting with the interns in which he put their jobs on the line.

"I had noticed in the past few days," said Whitelaw, "that they had not been doing the work they should. To remind them they were temporary employes only, I prepared termination papers with the names of each of them written on them. Then we had the meeting and I said, "There are two things you can do. You can continue to do your work fully or you can drop by my office on Friday and sign your termination notice.

Whitelaw was accompanied by David C. Johnston, personnel director, and his assistant Barbara T. Benson.

After Whitelaw left, Reed Waller, one of the interns, who is 22, said, "We were very upset - some of us were crying. We felt everyone was against us and that we were being unfairly accused."

The interns decided to resign en masse on Thursday but when they returned to work yesterday morning, three of them had changed their minds.

Said Lydia Barker, 22: "We are all going to school and we need the money." They were classified as GS-3s at an hourly rate of $3.81.

The three who had decided to stay on, and Braaten, arranged a meeting with Duffey because, Barker said, "We were afraid he was only hearing one side of the story." Said Waller: "We wanted to tell him we were being pressured to leave and accused of things we were not guilty of." The meeting was also attended by B. J. Stiles, deputy director for policy planning and public affairs, and Joy Evans, assistant to Elliott.

"Mr. Duffey gave us a very friendly reception," said Waller. "I did not come out of it intending to quit, but the more I thought of it was clear to me that Mr. Duffey had indicated he could'nt give us any help."

In his description of the meeting, Duffey said, "I told them I want to make it clear that nobody is going to be dismissed. But also I cannot guarantee they can always do assignments they will also operate under the most congenial of conditions."

The interns said Duffey, a former clergyman, spoke to them "on the trials and tribulations of life. He said that the fact the three of us were still at work proved that we were stronger than the others who had resigned."

Duffey said it had been made clear to the interns from the beginning that their responsibility would be primarily clerical. "We are saving substantial amounts of money by doing in-house work that formerly was contracted out - like mailings and clipping." The three interns agreed that the jobs had not been mis-represented to them.

According to Waller, "Mr. Duffey also told us that our work on the birthday party had been a very serious and illegal matter. He said what we had done - whistle blowing - was a good thing."

After the meeting with Duffey, the interns and Braaten went to lunch. When Waller, Barker and Andrea Hinds, 24, returned to the office they met with Elliott and Benson to discuss their duties. Elliott provided them a memorandum entitled "Your Responsibilities."

The memo instructed the interns that "in addition to any daily assignments you are given, your primary responsibility is to maintain the clipping files . . ."

Among the other instructions: "Everything in your office should be labeled and counted, i.e. spare envelopes, posters and other items stored in boxes. Nothing should be lying loosely about. A constant inventory should be kept of all materials in your office to avoid running out of kit covers, often-used releases, envelopes, etc. . ."

The interns said they discussed with Elliott her interpretation of their duties and felt there were discrepencies in Elliott's guidelines compared to Duffey's.

Waller said, "As we talked it just became increasingly clear that things were not going any better."

So they all walked out in mid-afternoon.

Duffey said the jobs "would all be refilled" and that any intern who quit "would be welcome back."