Lt. Col. Oliver L. North testified yesterday that he was part of an administration "fall-guy plan" in which he was to be the "scapegoat" shielding President Reagan and other high officials from the political and international repercussions of the covert Iran-contra operations that he managed.

The plan, which North and then-CIA Director William J. Casey drew up, called for North, a National Security Council aide at the time, to run secret operations from the White House -- without the knowledge or approval of Congress -- and to "take the hit" if those operations became public.

As part of his effort to conceal the roles of others, North testified yesterday, he shredded documents almost daily as the operations were "unraveling." He described an incident on the morning of Nov. 22 when he went about his shredding in the same office suite in which Justice Department officials were examining his remaining, undestroyed Iran files as part of a "fact-finding" scrutiny of the operation undertaken by Attorney General Edwin Meese III.

Although North said he would "walk past" the officials with a handful of documents and turn on the shredder in an adjacent room, a Justice Department spokesman said yesterday that the officials have a "firm recollection that while they were there, Col. North did not shred documents or even turn on a shredding machine."

Under questioning by Senate chief counsel Arthur L. Liman, North said he had changed his mind about remaining silent regarding the participation of top officials when Meese made him the principal target of a criminal investigation.

Toward the end of his third day of televised testimony before the Iran-contra committees, North again made an impassioned statement of his willingness to make sacrifices for Reagan, his commander-in-chief. Yet in an unprompted disclosure, he said that he had reviewed with Casey, Reagan's close friend and adviser, a memorandum intended for the president's approval, which contained a reference to the controversial diversion of proceeds from U.S.-Iran arms sales to the Nicaraguan rebels in February 1986.

In numerous conversations about the scheme, North said, the CIA director never told him the president was ignorant of it, as Reagan has asserted for the eight months since the scandal broke.

North also asserted that until late November, the last weekend he worked at the White House, he was operating under the belief that Reagan knew and approved of the 43-year-old Marine lieutenant colonel's activities on behalf of the contras -- including the diversion of Iranian funds to support them.

The high drama of yesterday afternoon's developments marked a sharp turn from the morning testimony, which featured the square-jawed North, with his wife, Betsy, seated behind him for the first time this week, issuing a challenge to the House and Senate panels to investigate their own role in the Iran-contra affair. He also called the hearings unfair and harmful to the interests of the United States.

But as Liman later drew out North by suggesting that he was an administration loyalist who had been misused by his superiors, North seemed to concur with his interrogator in describing his own apparent victimization in an administration cover-up.

"For whom were you going to be the scapegoat?" Liman asked.

"For whoever necessary. For the administration, for the president, for however high up the chain that they needed someone to say, 'That's the guy that did it, and he's gone, and now we've put that behind us and let's get on with other things,' " North replied.

North testified that he had met with Casey last November and that the director "agreed with my assessment that the time had come for someone to take the hit or the fall. He quite frankly did not think that I was senior enough to do that" and suggested national security adviser John M. Poindexter as a candidate. Rear Adm. Poindexter resigned on Nov. 25, the same day North was fired.

North said that "everything had gone right, according to plan, right up until 12:05 in the afternoon," of Nov. 25, when Meese held a White House news conference to announce that proceeds from the U.S.-Iran arms sales had been diverted to the Nicaraguan rebels and that a criminal investigation had begun.

According to North, his "mind-set" about the "fall-guy plan" changed radically when Meese talked about a criminal investigation during that news conference. Until then, North said, he had been destroying documents that revealed what other officials knew about his activities.

But after the Meese announcement his perspective changed, and he began to think about protecting himself. It was then, North said, that he decided to take home his personal spiral notebooks, which carried detailed jottings about the secret activities and people with whom he was in contact; he also removed several hundred pages of other documents "for one purpose, and that was to protect myself."

"There was probably not another person on the planet Earth as shocked as I was to hear that someone thought it was criminal. And I can tell you that shock was compounded when I heard later that there was to be an independent counsel, and further compounded when I was the only name in the appointment order for that independent counsel -- the only person on the planet Earth named in that appointment order," North said later.

North appeared less subdued earlier in the day when he read a formal "opening" statement that had been delayed two days as a result of legal maneuvering between the committees and North's counsel, Brendan V. Sullivan Jr.

North called the hearings a "strange process that you are putting me and others through." The committees, he said, "choose to provide distorted snippets of evidence." Of Congress, North declared: "You are to blame because of the fickle, vacillating, unpredictable, on-again, off-again policy toward the Nicaraguan democratic resistance." And, he charged, Congress "will not condemn itself."

Subsequent questioning by George Van Cleve, House deputy minority counsel, was interrupted by an impromptu demonstration by two men who jumped up from the visitors' benches in the rear of the Senate Caucus Room and unfurled a banner that appeared to say "Cocaine Smuggling." The two, who were hustled from the room by Capitol police, were charged with two misdemeanor counts.

Van Cleve, representing the House committee Republicans, offered North a chance to justify his own actions and beliefs, but concluded with a penetrating question that went to the issue of North's credibility as a witness. Noting that North had admitted to falsifying documents, as well as lying to Iranian representatives, retired Air Force major general Richard V. Secord, and Congress, Van Cleve asked if he was now telling the truth.

North said he was.

A major part of Liman's interrogation in the afternoon was aimed at shaking a North assertion that as a member of the president's staff, he had never circumvented the Boland amendment, which prohibited spending U.S. funds for military assistance to the contras beginning in October 1984.

North said that Casey, whom he described as a personal confidant though not his "boss," gave him the secret task that year of supporting the contras from the NSC, while the administration kept working on Congress to restore funding that had been cut off. Casey, depicted by North as an eminent lawyer, assured him that the Boland amendment did not apply to the president's staff.

North agreed with that view, and as Liman ticked off a number of North activities, the legality of which has been questioned, North defended that position.

The activities listed by Liman included proposing the option of sinking or seizing a ship carrying materiel to Nicaragua's ruling Sandinistas; carrying intelligence from the CIA and Department of Defense to the contras; and recommending the solicitation of funds from such foreign countries as Saudi Arabia to assist the rebels before such contributions for "humanitarian" purposes were permitted by Congress.

But as North insisted that these activities were never meant to be covered by Boland, Liman noted that former national security adviser Robert C. McFarlane had testified earlier that the NSC was covered by the law. North said McFarlane had never told him that.

North acknowledged to Liman that his covert operation in Central America was not run as other such CIA-led secret projects, which are undertaken with a signed authorization from the president, as required by law. But North continued to insist that such presidential approval was not needed for NSC-led covert operations, althought that did not mean the president did not know about them.

Liman then read a passage from the Tower review board saying the president had told the board that "he did not know that the NSC staff was engaged in helping the contras."

"Does that come as a surprise to you?" Liman asked.

"Yes," said North, who repeatedly said yesterday that he had authorization for everything he did.

North said he recalled showing Casey a copy of a memorandum with the scheme for diverting funds to the contras possibly as early as the February 1986 arms transactions, the first direct U.S.-Iran deal of that kind. He recalled going over it with Casey. He specifically remembered that the document included a reference to the diversion, and that it was in a form that was to go forward to the president for approval. Casey did not object to having it go to the chief executive, he said.

In the ensuing months, North had numerous discussions with Casey about the diversions, including some that centered on the political implications if the diversions were disclosed.

In early November, with Iran-contra operations being disclosed, Casey told North that "the time had come for someone to take the hit."

Casey, he said, was concerned, that the president not be damaged and North said he shared that belief.

Until Friday, Nov. 21, North said, he assumed that the president knew and approved of the diversion. On that day, with his programs collapsing and the Justice Department's fact-finding inquiry beginning, North said he tendered his resignation to Poindexter, his boss, and told him he had "cleaned up" his files -- saying he thought he had eliminated all documentation relating to the diversion.

In three days of testimony, North has not budged from his insistence that Poindexter also told him for the first time at that meeting that the president did not know of the diversion.

Two days later, on Sunday, Nov. 23, North was confronted by Meese with the diversion memo that Justice Department officials had found the previous day, and was questioned about it.

According to earlier testimony, North told Meese he did not "think" the president knew about the diversion.

After that meeting, North testified, he returned to his office and did more shredding. And on Nov. 24, he again offered his resignation -- and again shredded documents until the early hours of Nov. 25. A few hours later he learned for the first time that he had been dismissed and that a criminal investigation was starting.

On Nov. 25, when the president called North after North had been "dismissed" for his role in the diversion, North quoted the chief executive as saying, "I just didn't know."

"I do honestly believe that they expected that Ollie would go quietly," said North. "And Ollie intended to do so right up until the day somebody decided to start a criminal prosecution."

North, questioned yesterday about whether Meese had asked him to preserve all his documents, responded in the negative.

"Have you wondered why, if it was a good idea, that the president of the United States dismissed you because of it?"

"Let me make one thing very clear, counsel, this lieutenant colonel is not going to challenge a decision of the commander-in-chief, for whom I still work. And I am proud to work for that commander-in-chief. And if the commander-in-chief decides to dismiss me from the NSC staff, this lieutenant colonel will proudly salute and say thank you for the opportunity to have served and go."