LOS ANGELES (AP) _ A man who was tape recorded threatening to ruin the reputation of Michael Jackson is purportedly the father of a boy Jackson is accused of molesting.

''Certain things are going to have to come out and those two are not going to have any defense against me whatsoever,'' says the voice on the tape, broadcast Monday by a television station that said it had confirmed its authenticity. ''It's going to be a massacre if I don't get what I want.''

Jackson canceled a Singapore concert Monday, saying through spokesmen that he was too ill with a migraine headache to perform. But after tests, his doctor today pronounced him fit to continue his ''Dangerous'' tour.

''I apologize for any inconvenience I may have caused for my friends in Singapore,'' Jackson said in a tape recording played for reporters.

It was the third time Jackson postponed a show since launching the latest leg of his ''Dangerous'' tour last week, at the same time Los Angeles police revealed the entertainer was under investigation.

Police did not specify what the investigation was about, but various sources indicate Jackson was accused of molesting a 13-year-old boy. He denied any wrongdoing in a statement released last week.

KCBS-TV and CBS News on Monday broadcast excerpts of a recording of a telephone conversation allegedly made between the boy's father and stepfather. On it, a man's voice is heard making a threat involving Jackson and the boy's mother. The parents recently were involved in a custody dispute over the boy.

KCBS said the tape's authenticity had been verified by three sources it refused to identify and that police have a copy.

The voice identified as the father's never said during the conversation what he was seeking. KCBS said he was demanding a meeting with Jackson and that the recording was made in July, before the complaints were filed that prompted the investigation.

''This man (Jackson) is going to be humiliated beyond belief,'' the voice said. ''He will not believe what's going to happen to him - beyond his worst nightmares. He won't sell one more record.''

Asked by the stepfather if he was concerned about the impact the threat would have on his son, the reply was: ''It's irrelevant to me.''

The father's attorney didn't return phone calls seeking comment.

Jackson's private investigator, Anthony Pellicano, said the tape ''spells out everything to me, that this was an extortion attempt from the very beginning - that it was all planned.''

Pellicano claims a Beverly Hills dentist and sometime screenwriter accused Jackson of molesting his teen-age son and demanded $20 million to keep quiet. The dentist hasn't commented.

Also Monday, Los Angeles police officers traveled to Las Vegas, where they served a search warrant at the Mirage Hotel. They spent 30 minutes scouring through a three-bedroom suite frequented by Jackson, said Mirage spokesman Alan Feldman.

Police Lt. John Dunkin wouldn't comment on the search, but Feldman said the officers left empty-handed. Searches conducted earlier this month at Jackson's two Southern California homes netted videotapes and photographs, though the Los Angeles Times has quoted unidentified police sources as saying nothing incriminating was found.

Police and the Department of Children's Services were notified of the allegations by a therapist after the teen-ager detailed alleged sexual encounters with Jackson at his homes and during excursions to Las Vegas and Florida.

Back in Los Angeles, at a packed news conference held to promote an upcoming Jackson family TV special, the Jackson family defended its superstar member.

''I'd like to let the world know that I'm behind my son and don't believe any of this stuff that's being written about him,'' said Jackson's mother, Katherine Jackson.

Brother Jermaine said the family was planning a trip to visit Michael while he's on his world tour. The date was still being arranged, Jermaine said.

Absent from the event was Jackson's sister LaToya, who is estranged from the family. His other sister, Janet, was reported to have flown to Singapore last week to join him, along with Elizabeth Taylor.

In Singapore, concert promoters said Jackson collapsed after arriving at the National Stadium where he was to entertain a crowd of 40,000. He was rushed back to his hotel suite.

Several fans spent the night under his third-floor window.

Asked why she was waiting in the rain, Singapore student Jasmeet Kaur, 13, said, ''Michael is sick. If I get sick, I don't mind. I am here because I want to see him today.''