Fawn Hall, the former secretary of White House aide Oliver L. North, told Congress yesterday that she altered documents and helped shred a 1 1/2-foot-thick stack of sensitive papers at North's direction, and later smuggled incriminating copies out of the Executive Office Building after an FBI investigation had begun into the Iran-contra affair.

In often riveting testimony as the sixth week of congressional hearings began, Hall also acknowledged to the House and Senate select committees that she initially misled the White House counsel's office about the extent of the shredding. She further revealed that in the "panic" of events last November, she and North's deputy, Lt. Col. Robert L. Earl, had agreed not to disclose that they had removed documents from the National Security Council offices.

Pressed by Senate counsel Mark A. Belnick, an initially self-confident Hall gradually began to display the strain of a loyal secretary caught in the dilemma of wanting to defend a boss she admired while at the same time being required to tell the truth under a grant of immunity from prosecution. During more than two hours of testimony -- which will be continued this morning -- Hall at times appeared close to tears.

"I felt a little uneasiness when I was doing it, but I believed Col. North must have had a good reason, and I did what I was told," she said. Even after she had removed the documents from the White House -- thus putting herself in potential legal jeopardy -- she still felt in a "protective mode" toward North, she testified.

In addition to the drama her appearance provided, Hall supplied significant new information about North's operations, the role of his deputies, the mechanics of the NSC and, most startling, the almost desperate last-minute effort to destroy records.

She testified that:Contrary to the impression that North left with other key participants in the Iran-contra affair who have appeared as witnesses, he never received any telephone calls from President Reagan. As far as Hall knew, North did not have any "one-on-one" meetings with the president, although he once said he had been "at the residence." Logs of phone calls to North were destroyed, and committee investigators believe that those records may be lost forever. Also shredded were coded messages from North's network of operatives, and a number of electronic interoffice memos, many of which investigators have been able to reconstruct through copies or sifting through the NSC's electronic archives. North kept extensive notes on his daily activities in "thin, spiral reporter's notebooks." She believes she saw a stack of about 10 of the notebooks in the office of North's attorney, Brendan Sullivan. She also confirmed that North kept a log of what she believed were payments to contra officials in the back of another notebook. One of the Iran-contra affair's most critical documents -- a North memo believed to have been written in April 1986 outlining how funds diverted from the U.S. sales of arms to Iran could be used to support the contras -- was forwarded to national security adviser John M. Poindexter, who revised and returned it. The document, she confirmed, had been handled outside the usual NSC records system, was undated and was not logged because North "probably" wanted it handled that way.

But Hall was unable to solve a longstanding mystery as to whether it had eventually gone to the president, as recommended by the document. "I have to assume that it was typed for a reason and that it went somewhere," she said. She had heard rumors -- and believed them to be true -- that North kept cash in his office. And she testified that in June 1985 she had borrowed $60 from him in travelers' checks drawn on a Latin American bank for a weekend at the beach. "Make sure you pay back the money -- it's not mine," she recalled North telling her. She did, she said.

Washington attorney Thomas C. Green knew on Nov. 25 that Hall had smuggled documents out of North's office. The office at that point had been sealed by the NSC, following Attorney General Edwin Meese III's news conference disclosing the diversion of funds and announcing an FBI investigation. Hall said she was in Green's car when she removed the papers from her boots and the back of her dress and turned them over to North.

"Tom Green asked me if I was asked about the shredding what would I say. And I said, 'We shred every day.' And he said, 'Good,' " she told the committees.

Green's office did not return a phone call late yesterday.

A major part of Belnick's questioning yesterday centered on the attempt by North last November to alter a series of sensitive NSC documents drafted by North in 1985 and sent to then-national security adviser Robert C. McFarlane.

The documents dealt with North's proposal to sink or seize a ship carrying arms to the Sandinistas, future contra operations and how money had been spent in the past on weapons. All of this occurred at a time when Congress had prohibited any U.S. military aid or intelligence assistance. In August 1985, when members of Congress raised specific questions about North's activities, the NSC general counsel at that time, Cmdr. Paul B. Thompson, found them in the files and told McFarlane they could cause problems.

McFarlane had North review them and according to McFarlane's testimony to the committees last month, North proposed making changes and kept a list of these "problematical" documents taped to his desk.

According to Hall, it was these documents, normally kept in special custody under the NSC's most secure "System IV" classification, that North asked her to alter in the late afternoon of Nov. 21. At the time, she said, she was unaware that Meese had arranged for his Justice Department associates to begin reviewing NSC documents the next morning.

Hall told the committees it was the first time she had ever been asked to make changes in original "System IV" documents, and then to destroy the originals and replace them with the copies in her own backup files. But she said it was "a policy of mine not to ask questions; I had no right to question him {North}."

At one point she said that she did not read the changes that she had been asked to make -- an assertion that gained credibility from the fact that in one case she deleted all but two innocuous paragraphs from what formerly had been a two-page memo, but inadvertently left references to several deleted attachments, including one to "military equipment requirements."

Overall, according to Belnick, the deletions removed suggestions of North's active role in contra assistance and White House knowledge of it and left the impression that North was essentially a passive onlooker.

As Hall described the scene, she was altering papers while North was pulling documents from a five-drawer safe that she said had contained his personal papers. Earl, she said, was "carrying files," apparently at North's direction. During the shredding, however, the machine clogged, and Hall put in a call to the crisis management center. It dispatched a man called "J.R.," she said, and he fixed the shredder.

Over the next two days, the diversion memo was discovered, and North told Hall he had had a "lousy" weekend. The next day, Nov. 25, North told her the president had fired him. When she realized he was serious, she began to cry, she said.

After North left and the NSC sealed the office, Hall said she realized with "panic" that she still had in her files a number of the altered documents as well as messages similar to those that had been destroyed. She called North, she said, using a whisper to "convey to him that I found documents," and "insisted he come back."

Earl, also present, was going to take some notes out, she said. But she said she told him, "No, you shouldn't have to do this, I'll do it." North, accompanied by Green, arrived after she had finished stuffing the documents in her boots and the back of her dress.

An NSC official checked the men's briefcases to make sure no documents were taken from the sealed office. But Hall's ploy was not discovered. The two men, North and Green, told her to keep them hidden until they reached Green's car.