Lt. Col. Oliver L. North's six days of testimony came to an extraordinary end yesterday as the two chairmen of the Iran-contra select committees delivered stinging rebukes to North and the Reagan administration for conducting a secret foreign policy in defiance of democratic principles and repeatedly lying about it to Congress and the American people.

The former White House aide, whose engaging manner during his appearance transformed him into a new American celebrity, sat stiffly as Rep. Lee H. Hamilton (D-Ind.) told him calmly and directly: "I do not see how your attitude can be reconciled with the Constitution of the United States."

North maintained the same solemn demeanor, at times resting his chin on his folded hands and furrowing his brow slightly, as Hamilton told him:

"In your opening statement you said that these hearings have caused serious damage to our national interests. But I wonder whether the damage has been caused by these hearings or by the acts which prompted these hearings. I wonder whether you would have the Congress do nothing after it has been lied to and misled and ignored. Would we in the Congress then be true to our constitutional responsibilities? Is it better under our system to ignore misdeeds or to investigate them behind closed doors, as some have suggested? Or is it better to bring them into the open and try to learn from them? I submit we are truer to our Constitution if we choose the latter course."

North's final hour on the witness stand added even more drama to what already had been an emotionally draining week for North, the two committees, and millions of Americans watching North's revealing and damaging testimony on television.

North listened silently to Hamilton for 35 minutes and then heard Sen. Daniel K. Inouye (D-Hawaii), who lost his right arm in World War II, salute him as a fellow soldier but say that "as one who participated in the making of this new American hero, I've found certain aspects of your testimony to be most troubling."

Inouye spoke sternly and with more emotion than Hamilton, who delivered his remarks in slow, patient, schoolmasterly tones. Inouye expressed concern about what lessons young Americans would draw from North's testimony and the favorable public reaction that has resulted.

He noted that North, as a student at the U.S. Naval Academy, promised to obey an honor code that prohibited lying or cheating and said:

"When the colonel put on his uniform and the bars of a second lieutenant, he was well aware that he was subject to the Uniform Code of Military Justice. It's a special code of laws that apply to our men and women in uniform. It's a code that has been applicable to the conduct and activities of Col. North throughout his military career, and even at this moment. And that code makes it abundantly clear that orders of a superior officer must be obeyed by subordinate members -- but . . . it must be the lawful orders of a superior officer."

Inouye suggested that North, as a military officer, had an "obligation" to disobey unlawful orders -- a principle, Inouye said, that the U.S. government considered important enough to insist that it be applied at the Nuremberg trials of Nazi war criminals. The mere mention of that post-World War II court provoked an angry reaction from North's attorney, Brendan V. Sullivan Jr.

"I find this offensive," Sullivan shouted. "I find you engaging in a personal attack on Col. North, and you're far removed from the issues of this case. To make reference to the Nuremberg trials, I find personally and professionally distasteful, and I can no longer sit here and listen to this."

Inouye cut off Sullivan. "You will have to sit there, if you want to listen," he said, and then continued with his remarks.

Nothing in North's last day prepared those inside the Senate Caucus Room or watching nationally on television for the emotional finish. For much of the morning, the committees had wrangled along partisan lines over whether to permit North to give a "slide show" talk similar to ones he made at contra fund-raising events while he worked on the staff of the National Security Council -- a dispute resolved with a compromise. North got to give his speech by holding his slides but not projecting them.

One significant new detail did emerge. North mentioned that he had turned down a "bribe" of $1 million from Iranian arms dealer Manucher Ghorbanifar, who offered it as an "incentive" to make certain Iran arms sales went forward.

Otherwise, it was mostly a day for speeches by committee members. Rep. Louis Stokes (D-Ohio), for example, pointedly told North that black Americans had learned to work within the legal system to change unjust laws and that he expected North to follow the same "rule of law."

The first hint of something extraordinary to come was shortly after 3 p.m., when Sen. Warren B. Rudman (R-N.H.), vice chairman of the Senate panel, took the microphone to announce that "something has occurred that . . . has been so disturbing to me that I wanted to say what I'm going to say, probably over the chairman's objections."

Speaking with obvious anger, and seeming to try to control his emotions, Rudman said: "We received some calls in the committee and our offices over the last 72 hours of ugly ethnic slurs against our chairman, and other kinds of calls that were extraordinarily insulting to the members of this committee."

Rudman added: "The chairman was recommended for the Congressional Medal of Honor for assaulting two German machine gun nests in Northern Italy, and then falling on the third one which was destroying his company, when he lost his arm which he left on that battlefield in Italy."

Noting that Inouye holds the nation's second-highest military decoration, the Distinguished Service Cross, Rudman said of him:

"He is one of the greatest men I ever have known, and the country ought to know the kind of leadership the Senate chairman exerts -- and for all Americans to condemn the kind of ethnic slurs that have no place in America."

North, in uniform as usual and wearing his own campaign ribbons signifying medals for bravery in battle, replied softly: "I fully agree, Mr. Rudman."

Inouye asked if North wished to make a final statement. The colonel, speaking in the same manner that captivated millions watching him since he first took the stand a week earlier, was brief. He said, "I would simply like to thank the American people who have responded with their good wishes, their support, their prayers through what has been for me and my family a long a difficult ordeal. I thank them for that, and salute them. That is my statement, sir."

Those were North's last words before the committees. The rest belong to the respective chairmen.

Hamilton was first.

He praised North for what he called his good intentions and for having shared in "agonizing choices" that had confronted President Reagan.

"Yet what strikes me," Hamilton said, leaning forward as if gently lecturing the officer whose wife, Betsy, sat behind him, "is that despite your very good intentions, you were a participant in actions which catapulted a president into the most serious crisis of his presidency, drove the Congress of the United States to launch an unprecedented investigation and I think probably damaged the cause, or the causes, that you sought to promote."

It was not his task to judge North, nor the committees', Hamilton said in the same patient manner, and he spoke sympathetically of North having been caught in the middle "in a struggle between the Congress and the president over the direction of American foreign policy."

"What bothers me," Hamilton explained, were two things: first policy and then process.

"The president has acknowledged that his policy, as implemented, was an arms-for-hostage policy. And selling arms to Iran in secret was, to put it simply, bad policy.

Hamilton said it had "contradicted and undermined" longstanding and widely articulated U.S. public policy and damaged American credibility.

"A great power cannot base its policy on an untruth without a loss of credibility," he said, still speaking directly to North. "Friendly governments were deceived about what we were doing. You spoke about the credibility of U.S. policy in Central America, and you were right about that, but in the Middle East, mutual trust with some friends was damaged, even shattered."

Turning to process, Hamilton spoke of concerns over the inherent conflicts between covert operations and democratic principles, about accountability for actions undertaken in secret, and said the secret operations in which North was involved had not met the test of accountability.

Turning to North's personal views, Hamilton said: "I'm bothered by your comments about the Congress. You show very little appreciation for its role in the foreign policy process. You acknowledge that you were 'erroneous, misleading, evasive and wrong' in your testimony to Congress. I appreciate, sir, that honesty can be hard in the conduct of government. But I am impressed that policy was driven by a series of lies -- lies to the Iranians, lies to the Central Intelligence Agency, lies to the attorney general, lies to our friends and allies, lies to the Congress, and lies to the American people."

He quoted Thomas Jefferson: "The whole art of government consists in the art of being honest."

Inouye's closing remarks had a different kind of emotional power. They represented an exchange between two decorated veterans, both of whom sustained wounds in two American wars, but who viewed public policy questions differently. Noting that Americans have "the right to dissent, the right to criticize the leaders of this government," Inouye also quoted Jefferson: "The spirit of resistance to government is so valuable on certain occasions that I wish it to be always kept alive."

North neither objected nor challenged what had been said. When it was over, he left to begin the difficult role Inouye had assigned him, the "heavy" burden of being "a new American hero." North made one quick stop before the cameras outside the hearing room, again thanked the American people for their support, and departed.