In a riveting and emotional confrontation broadcast live from a spellbound Senate hearing room, law professor Anita Hill yesterday vividly recounted the sexual harassment she said she suffered from Supreme Court nominee Clarence Thomas. Thomas angrily denied the charges of his former aide and denounced the confirmation process as a "high-tech lynching."

The two accounts -- flatly contradictory, impossible to reconcile -- left the senators with a painful choice.

"They cannot both be telling the truth, said Sen. Patrick J. Leahy (D-Vt.), who questioned both witnesses.

Sen. Herbert H. Kohl (D-Wis.) agreed. "Clearly, one is lying and one is telling the truth, but I can't say who."

In an opening statement and under lengthy questioning from senators, Hill alleged that Thomas pressured her repeatedly to go out with him, bragged of his sexual prowess, and talked to her luridly about pornographic movies and sex. Thomas, in a statement that opened the morning session, and in an angry rebuttal of Hill's charges last night, denied it all categorically and "unequivocally."

The evening session was the emotional high point of a day that seemed to drain both the senators and the witnesses. A clearly infuriated Thomas told the Senate Judiciary Committee its hearing was a "national disgrace. And from my standpoint, as a black American, as far as I'm concerned, it's a high-tech lynching for uppity blacks who in any way deign to think for themselves, to do for themselves, to have different ideas . . . . You will be lynched, destroyed, caricatured by a committee of the U.S. Senate rather than hung from a tree."

The two Yale-educated lawyers, launched from similar rural backgrounds, were remarkable contrasts behind the felt-covered witness table. She was poised and unflappable, and answered every question put to her. Thomas was fiery; when he returned for the night session, he stunned the senators by saying he had not listened to Hill's testimony. "I would not want to, except being required to be here, dignify those allegations with a response," he said.

He is to return this morning as the extraordinary hearing, which appears likely to stretch through the weekend, resumes.

The committee convened the hearings -- which at several points threatened to degenerate into a brawl between Republicans and Democrats on the panel -- after the public disclosure of Hill's allegations about Thomas.

Hill, 35, went much further yesterday, resulting in an unprecedented and in many ways distasteful public session in which the committee heard testimony, not about natural law and constitutional adjudication, but about pornographic depictions of large-breasted women having sex with animals.

The Senate Caucus Room was silent as Hill testified that the Supreme Court nominee repeatedly told her she should go out with him, bragged to her about his physical endowment, graphically recounted pornographic movies he had seen and finally warned her that it would ruin his career if she ever revealed his behavior.

"Telling the world is the most difficult experience of my life but it is very close to having to live through the experience that occasioned this meeting," said Hill, who was an aide to Thomas, 43, at the Department of Education and the Equal Employment Opportunity Commission.

Unrattled throughout her testimony, Hill sought to explain her decision to follow Thomas to the EEOC despite his alleged behavior at the Education Department, her failure to come forward earlier with the explosive allegations, and her continued contact with the man who allegedly harassed her after she stopped working for him.

Thomas, who spoke to open the hearing and then left the room while Hill testified, sounded at times as if he were ready to withdraw. "Enough is enough. I am not going to allow myself to be further humiliated in order to be confirmed," Thomas said yesterday morning.

Since FBI agents first informed him of the allegations when they came to interview him on Sept. 25, Thomas said, "I have been racking my brains and eating my insides out trying to think of what I could have said or done to Anita Hill to lead her to allege that I was interested in her in more than a professional way, and that I talked with her about pornographic or X-rated films."

The allegations have transformed the political landscape of the Thomas nomination, and possibly the future of the confirmation process.

Democratic supporters, whose constituency seemed to have little interest in the nomination and who provided Thomas with what had appeared to be a comfortable margin of victory, suddenly found themselves facing the potential wrath of constituents who believe Hill's account.

White House officials, worried and cautious throughout the day of hearings, were jubilant at the conclusion of the evening session. "Judge Clarence Thomas's message tonight was a powerful testament to his integrity and and character," said the administration statement. "He should be confirmed to the Supreme Court. His testimony speaks for itself."

Harvard Law School professor Charles Ogletree, one of Hill's lawyers, said last night that she "was tired, depressed, she feels beaten. It's a very tough process."

Among the expected witnesses today is another former employee, Thomas's director of public affairs at EEOC, Angela Wright, who was fired from her post and is now an editor at the Charlotte (N.C.) Observer, has told committee lawyers that Thomas repeatedly asked her out, inquired about her breast size, and showed up uninvited at her apartment.

Thomas's supporters have said Wright's story is not credible and attacked her character.

Also appearing will be three witnesses in whom Hill said she confided her problems with Thomas at the time they occurred in the mid-1980s, as well as former co-workers of both Hill and Thomas who have strongly defended Thomas's propriety and criticized Hill.

Three weeks ago, Thomas's confirmation appeared all but assured even as the committee split 7 to 7 on the nomination. All that changed dramatically last weekend, when news reports revealed Hill's allegations. In the uproar that ensued, the Senate was forced to postpone the vote, which had been scheduled for last Tuesday, and reopened the hearings.

Thomas entered the room to cheers. Supporters lined the hallways, wearing T-shirts that proclaimed, "Take a stand for righteousness and for Thomas."

With his wife, Virginia Lamp Thomas, and his chief Senate advocate, John C. Danforth (R-Mo.), sitting behind him, Thomas said he "cannot imagine anything that I said or did to Anita Hill that could have been mistaken for sexual harassment." But he said that if any of his comments or actions had been "misconstrued, by Anita Hill or anyone else, to be sexual harassment, then I can say that I am so very sorry and I wish I had known."

The day's events, with the last-minute release of affidavits and bitter wrangling over the scope of questioning and other issues, belied Committee Chairman Joseph R. Biden's opening description of the event as a "fact-finding" mission, not a partisan trial.

Biden reflected the sense of many of his Senate colleagues that the body itself was on trial for its decision not to fully investigate Hill's allegations before they were reported in news accounts. He defended his handling of Hill's allegations, saying he could not have "forced Professor Hill against her will into the blinding light which you see here today."

But, he said, "I'm deeply sorry that our actions in this respect have been seen by many across the country as a sign that this committee does not take the charge of sexual harassment seriously. We emphatically do."

Sen. Dennis DeConcini (D-Ariz.), the sole Democrat on the committee who voted for Thomas and who reaffirmed his belief in the nominee even as Hill held a news conference on Monday, sounded less sure yesterday.

He told Hill her allegations described "god-awful things" and said after the hearing broke up last night that her testimony was "impressive," although not without "some flaws."

DeConcini said that when he initially read the FBI report, "I came down on the side of Thomas. Now I feel I need to listen" to further evidence.

Thomas seemed to try to turn the tables on Hill, calling it "particularly troubling that she never raised any hint that she was uncomfortable with me" at any point during the last decade.

He expressed a sense of betrayal by Hill, "a person I have helped at every turn in the road since we met," and said it was impossible to endure such charges.

"No job is worth what I've been through -- no job," he said. "No horror in my life has been so debilitating. Confirm me if you want. Don't confirm me if you are so led. But let this proceess end."

Hill, the youngest of 13 children, grew up in an Oklahoma farm family and went on to Yale Law School and a job in civil rights, a background remarkably similar to Thomas's. With her parents, five of her siblings, and a battery of lawyers sitting behind her, Hill said she felt "angry and disgusted" by Thomas's conduct, which she described as "unbefitting an individual who will be a member of the court."

She said the combination of his asking her out and his graphic sexual comments "really made me feel helpless in a job situation."

She said that about three months after she started to work for Thomas as his attorney-adviser in the Education Department's Office of Civil Rights he asked her to go out with him.

Hill said she thought that was "ill-advised" and told him so, but he did not drop the subject and "pressed me to justify my reasons for saying no to him."

Next, she said, Thomas started to turn work discussions to the topic of sex, with "vivid" accounts of pornographic films involving such material as "women having sex with animals," "films showing group sex or rape scenes," and pornographic material showing "individuals with large penises or large breasts involving various sex acts." One such person, she remembered Thomas telling her, was known as "Long Dong Silver."

After she followed him to the EEOC, she said, "he referred to the size of his own penis as being larger than normal" and recounted "the pleasures he had given to women with oral sex."

Perhaps the "oddest episode," she said, took place while Thomas was drinking a Coke in his office at the EEOC. "He got up from the table at which we were working, went over to his desk to get the Coke, looked at the can and asked, 'Who has put pubic hair on my Coke?' "

Sen. Arlen Specter (R-Pa.) had the unenviable task of being the Republican's designated questioner of Hill. A former Philadelphia prosecutor, Specter hammered away at inconsistencies and weaknesses in Hill's accounts in the manner of a lawyer seeking to discredit the testimony of a hostile witness.

He pointed out that she did not tell the FBI agents who questioned her last month some of the more graphic details she recounted in a statement to the committee or revealed yesterday, such as the Coke can incident. He repeatedly noted that the statute of limitations on sexual harassment civil lawsuits is six months because such charges are so hard to defend against.

Hill acknowledged the accuracy of logs produced by Thomas supporters that show 10 telephone calls from her to his office mostly between 1984 and 1987. She repeated her assessment of use of the phone calls to undermine her account as "garbage."

She said she called three times to help arrange a civil rights conference in Tulsa. She said his secretary, Diane Holt, was a friend and when Holt told her about Thomas's marriage, "I did say 'congratulate him.' "

Hill disputed a Washington Post account of an interview Monday night in which she said that "if these are messages to him from me, these are attempts to return phone calls." Yesterday, she acknowledged initiating some calls.

In an effort to damage Hill's credibility, Republicans released an affidavit from a Texas lawyer and consultant who said he met Hill in Washington during the early 1980s and that she berated him at her going-away party, before she left town to teach law in Oklahoma, for having led her on.

The consultant, John N. Doggett III, a friend of Thomas's, said he was "stunned" by Hill's remarks and was left "feeling that she was somewhat unstable." He speculated Hill had developed "fantasies about my sexual interest in her."

Hill dismissed Doggett's allegations, saying she had little recollection of knowing him and that she had no romantic interest in him.

The evening session took on an edge of nastiness as Thomas for the first time turned critical of Hill, saying there was "friction" between the "aloof" Hill and other staffers that "usually involved her taking a firm position and being unyielding to the other members of the staff and then storming off and throwing a temper tantrum."

Thomas literally pointed his finger at the committee as Sen. Howell T. Heflin (D-Ala.) repeatedly asked what motive Hill might have in accusing him. "Senator, I am incapable of proving the negative," he said. "It did not occur."

Later, Thomas for the first time suggested a possible motivation, saying he thought Hill had been unhappy when, after the move to the EEOC, she found herself having less contact with him and was not promoted to a position as his chief of staff.

"Her work was good," Thomas said. "The problem was it was not as good as some of the other members of the staff."

Thomas said Hill would at times invite him into her house when he drove her home and he would "stop in and have a Coke or a beer and continue arguing about politics or something for half an hour."

Heflin expressed displeasure that Thomas had chosen not to watch Hill's testimony, telling him "that puts us somewhat in a more difficult position." It was a situation reminiscent of the frustration among some panel members when they learned that Thomas had not reread a controversial anti-abortion article that he had praised or looked at a White House report that he had signed.

Hatch took Thomas point by point through Hill's specific allegations, which a bristling Thomas denied one by one. "This is embarrassing for me to say in public but it has to be done," Hatch said as he questioned Thomas about the Coke can incident.

Hatch asked Thomas if he had ever talked to Hill about "Long Dong Silver."

Thomas shook his head sadly. "No, senator."