A former congressional page, a central figure in the investigation of allegations that congressmen had sex with teen-age pages, has over the past 10 days given conflicting stories about his alleged experiences and on Thursday failed parts of a lie-detector test administered by the FBI.

Leroy Williams Jr., the 18-year-old former Capitol Hill page who has claimed firsthand knowledge of illicit sexual activities by members of Congress, acknowledged in an interview with The Washington Post that he exaggerated parts of his story when he first told it publicly.

Through 10 hours of interviews with The Post last week in his home in North Little Rock, Ark., and, he said, during closed-door sessions here yesterday with staff members of the House ethics committee, Williams stuck to his basic story.

While serving as a page here last year, he maintains, he had sex with three congressmen, and on two other occasions, arranged sessions with prostitutes--once for a senator and once for a congressional aide.

Two of the three congressmen, the senator and the congressional aide cited by Williams have told The Post that Williams' allegations were totally untrue. One of the congressmen whose name was mentioned by Williams could not be reached for comment yesterday.

Bob Scott, Williams' lawyer, and an FBI source said yesterday that the lie-detector test indicated that Williams was not telling the truth when he was asked specifics of his alleged sexual relations with congressmen.

Williams' allegations and those of another former page have touched off investigations by the Justice Department and the ethics committee to determine if members of Congress had sex with minors or improperly promised favors to pages in return for sex. Homosexual activities are not the focus of the probe.

In the first public airing of his allegations, a televised interview broadcast June 30 by CBS News, Williams said he had sex with one congressman three times, twice in the congressman's office and "one other time was at his apartment. Or you know, he said it was his apartment. I heard it wasn't because I had heard that he lived somewhere else, but this was at the Watergate."

In a taped interview with The Washington Post three days later in the tiny living room of his home, however, Williams said he lied when he said there was a third sexual liaison with the congressman and that it occurred at the Watergate.

"You associate congressmen with big things; the Watergate's something big in D.C. anyway you look at it," Williams said. "And so I guess it was just . . . the evil part of me saying, hey, here's something that sounds good, you know.

"If you can stick him at the Watergate and make it sound like you know something went on at the Watergate, then that'll make the story sound twice as good," Williams said.

He said he did meet the congressman once for a drink at the Peacock Lounge at the Watergate Hotel, but he did not have sex with him on that occasion.

Later in the same interview, Williams said, "My entire interest wasn't in making the story sound good, but along with the truth, some of the things that I said were said in order to make sure the heat was put on them congressmen , not me. It was a self-protective measure."

Williams first gave the interviews on the condition that his name not be used, but agreed yesterday to make their contents public.

Williams, one of seven children from a North Little Rock family, started as a page in late June 1981, the first page appointed by Rep. Ed Bethune (R-Ark.), and was elevated to one of two Republican page supervisory positions by Sept. 1, 1981.

However, four months later Williams abruptly resigned as a page.

Rumors about his sexual life on the Hill and elsewhere surfaced later when his former landlady found in his apartment another page's billfold and pictures described as sexually oriented material.

Shortly after Williams left Washington, Capitol police began an investigation into the social activities of several pages, including Williams. The D.C. police also investigated Williams in connection with a 1980 Camaro automobile that another page had reported missing and that later was found in Tuscaloosa, Ala., according to D.C. police records.

When Williams, wearing khaki pants and an oxford shirt, spoke to a throng of reporters outside a House annex building in Southwest Washington yesterday, a reporter asked him if he had stolen the car.

"No, I did not," Williams replied.

Charges were never filed in connection with the incident, police records indicate.

Lawyer Scott, who accompanied Williams to Washington, said yesterday that Williams, in all of his allegations, is "either telling the truth or he does not know the wide difference between truth and fiction. I am convinced he is telling what he believes to be the truth."

Informed that Williams had told The Post that he had sex with the congressman twice and not the three times Williams said in the CBS interview, Scott said: "I don't know how to explain that inconsistency."

Scott, emphasizing that he was not trying to justify inconsistencies in some of Williams' accounts, said it is understandable that under the present intense pressures on Williams, the former page could get confused about the sequence and details of events that happened months ago.

Scott said Williams has no ulterior motive to implicate the particular congressmen he has alleged to have had sex with or to fabricate his basic story. "What the hell is the motive?" Scott said.

Williams told reporters yesterday he never intended to divulge his alleged sexual relationships with the congressmen. "It was something I wanted to leave behind," Williams said.

But, Williams said, he decided to go public with his accusations when his name began to be mentioned on Capitol Hill in connection with alleged drug use. "If the people of America are going to hear this story, they need to hear my story," Williams said.

Asked if he was involved with drugs while a page, Williams said he used drugs "on a limited social basis," but he was not a drug dealer.

Both Williams and Scott attacked the objectivity of the polygraph test administered by the FBI in Little Rock Thursday before Williams and his lawyer flew to Washington.

Scott said the FBI examiner told him that Williams was "lying from beginning to end. He said there's not a bit of truth about any of the allegations. In all my experience" as a lawyer, Scott said, "I never had a polygraph examiner say that someone who is being examined is lying."

Lie-detector results are not admissible in court as evidence because of questions about their reliability. However, an FBI official said they are used as an investigative tool and "it is given heavy consideration" by the investigators.

"It's not as though [the investigation] is ending because he failed," the FBI official said. "It's just another development."

The FBI official said agents so far have been unable to substantiate the allegations.

The investigation began when another page went to the Justice Department with allegations of sexual misconduct by congressmen. However, except for one instance in which the page claimed he turned down a sexual proposition from a congressman, the page could not provide investigators with firsthand knowledge of the alleged improprieties, according to sources familiar with his statements.

The page told investigators that Williams might have direct information, and Williams subsequently was interviewed by the FBI in late June.

Several other conflicts emerged in The Post's interviews with Williams:

* In the case of the alleged date with a male prostitute that Williams says he arranged for a senator, Williams first told The Post that the senator discussed it with Williams at a congressional reception.

In an interview a day later, Williams said the senator approached him in a Capitol Hill corridor.

In a separate interview broadcast by CBS, Williams said the senator first contacted him by telephone while Williams was at work.

Williams said also he arranged for the prostitute to meet the senator in the Watergate apartment of a Williams friend. CBS News has reported that the prostitute mentioned has said in an interview that he had sex with Williams, not the senator.

Williams told The Post that on several occasions he had sexual relations with prostitutes from the same modeling service he allegedly called on behalf of the senator.

* After he allegedly had sexual relations with a second congressman, Williams said, the congressman said he would recommend Williams for a job on a House committee. Williams said a committee aide told him later that his name had been submitted for a job.

In an interview with The Post this week, however, the aide said she never discussed any job with Williams and was unaware of his name ever being submitted to the committee.

* The congressman who allegedly offered him the committee job was also a member of that panel, Williams said in his first interview with The Post. However, a check by The Post revealed that the congressman was not a member of the committee.

When confronted with his inconsistencies during the last interview in North Little Rock on July 3, Williams told The Post that he had decided not to make up anything else.

"If the truth isn't there," Williams said, "then I can be ruined for the rest of my life, and also other people can be ruined for the rest of their lives. And I don't want that to happen."