NEW YORK, MARCH 25 -- Interrupting the ninth day of increasingly contentious jury deliberations, prep school graduate Robert Chambers pleaded guilty today to first-degree manslaughter in the death of 18-year-old Jennifer Levin, who died during a sexual encounter in Central Park.

Chambers, 21, told authorities in a videotaped confession that he strangled her accidentally when she hurt him. He stood staring at the floor today as state Supreme Court Justice Howard E. Bell asked whether he intended to injure Levin seriously and "thereby caused her death."

"Looking back on everything, I have to say 'yes,' " Chambers said. "It breaks my heart to have to say that." The judge repeated the question three times before Chambers replied with the required, unqualified yes.

As part of the agreement, he also pleaded guilty to one count of second-degree burglary stemming from an earlier case. Bell said Chambers will be sentenced to two concurrent five-to-15-year sentences and allowed him to spend tonight with his family before surrendering at 10 a.m. Saturday.

Chambers had been charged with second-degree murder in the August 1986 slaying and could have been sentenced to 25 years to life. The minimum sentence for the manslaughter charge is two to six years. He will be eligible for parole in five years.

"I'm disappointed that they did not reach the verdict we wanted," Ellen Levin, the victim's mother, said at a news conference late this afternoon. "I understand the pressures they were under. There needs to be more balance in our justice system. Evidence of his past should have been allowed. If only the jurors knew he wasn't the lily-white altar boy he was painted.

"I'll feel better when Robert Chambers is in jail," she said.

Manhattan District Attorney Robert M. Morgenthau said his office "agreed to a plea only after a careful review of all the circumstances in this case. The Chambers jurors deliberated for nine days, the longest deliberation in recent New York County history. They released one note today reporting that they were at an impasse, and it was apparent . . . that a mistrial was likely."

One juror, a young banker who identified himself as Michael Ognibene, said the jury, which was instructed by the judge to deliberate on the lesser charges of first- and second-degree manslaughter and criminally negligent homicide as well as second-degree murder, never got beyond the murder charge.

The jurors, whose names were sealed by the court at the onset of the trial, took four votes. The first was 8 to 4, not guilty. The second, 9 to 3, not guilty. The third, taken last night, was 9 to 3, guilty, or a complete reversal. This afternoon, the juror said, they voted 7 to 5, not guilty.

Morgenthau said that he consulted the Levins before agreeing to take the plea and that they consented because they said they could not withstand a second trial. Testimony in this trial took 10 weeks.

Prosecutor Linda Fairstein said "yesterday one juror, and today two more jurors, sent notes out saying they could not go on because of mental and physical strain." There was no indication of how the jury was divided, but Fairstein said defense attorney Jack T. Litman initiated plea negotiations.

One young woman on the jury of four men and eight women wept as the judge guided Chambers through the plea bargain. As the jurors filed from the courtroom afterward, Ognibene reached out and shook Chambers' hand.

Chambers and his parents left by a side door and were escorted to a waiting brown Cadillac. A police cruiser led the car away from a crush of cameras and reporters.

The trial had played to New York audiences like a soap opera, and crowds waited in line at the courtroom each day. When the judge released copies of the videotaped confession, in which Chambers demonstrated the logistics of his encounter with Levin, portions were broadcast on every local news program.

Chambers and Levin, children of wealthy Manhattanites, were part of a clique of privileged prep school alumni, and the public seemed to devour details of their self-indulgent life style. Their circle of friends met often at Dorrian's Red Hand, a trendy East Side bar, smoking and drinking into the wee hours. The two were seen leaving Dorrian's at 4:30 a.m. the day Levin died.

Chambers, who did not testify, said in the confession that he and Levin engaged in rough sex play in a patch of grass behind the Metropolitan Museum of Art. He said Levin had tied his hands with her panties and "molested" him.

Her partially clad body was found by an early morning bike rider. Chambers initially denied that he had killed her and later maintained that it was an accident. He said he locked his arm around her neck and yanked her off of him after she deliberately hurt him by squeezing his testicles.

In her summation, prosecutor Fairstein asked the jurors, most of whom were in their 30s, to compare the photographs of Levin taken at Dorrian's hours before her death with pictures taken the next morning at the city morgue.

"All those photographs of her speak about the violence of her murder," Fairstein said. "The injuries scream to you . . . that Jennifer Levin's death was intentional.

"Approve of her life style or not," Fairstein said, "she had every right to live out her life."

Fairstein told the jury that Chambers' motive could have been irrational and that he might have been angered by a failure to perform sexually. But "no motive we can offer you could justify the taking of Jennifer Levin's life," Fairstein said.

Members of victims' rights groups watched Litman's defense tactics because of his reputation for attacking victims' characters. He was criticized when he unsuccessfully sought before trial to get Levin's diary.

But Litman called only five witnesses and kept his presentation low-key. His soft-spoken summation emphasized the prosecution's failure to establish a motive. "He never intended to kill Jennifer Levin or even to hurt her," Litman said. "He was in a frenzy, reacting to the pain that she caused him, the kind of pain that is clearly not going to dissipate immediately."

Litman recalled the testimony of Elizabeth Shankin, a friend of Levin and Chambers, who said Levin had aggressively pursued Chambers. Shankin said Levin had confided that, on previous dates with Chambers, they engaged in "the best sex I ever had."

Chambers had just returned to New York from a drug-rehabilitation clinic in Minnesota. Earlier, he had been expelled from Boston University. Levin, who graduated from Manhattan's Baldwin School the previous summer, was planning to enter college in Boston soon.