One of the 12 persons scheduled to go on trial next week in the beating death last October of Catherine L. Fuller pleaded guilty to second-degree murder yesterday as part of a plea bargain under which he will testify for the prosecution.

The plea by Calvin L. Alston, 20, of 1175 Third St. NE, represented a major break for prosecutors, occurring only days before the expected start of what is believed to be the largest murder trial in D.C.'s history.

Alston, entering the guilty plea before Superior Court Judge Fred B. Ugast, admitted that he had beaten and kicked Fuller in addition to serving as a lookout.

Assistant U.S. Attorney Jerry S. Goren yesterday told Judge Ugast that Alston will implicate most of the 11 defendants in Fuller's death. Until yesterday's plea bargain, the government's case included few witnesses to the Oct. 1, 1984, slaying of the 49-year-old mother of six. Another defendant who pleaded guilty to manslaughter and robbery in the case has agreed to testify for the prosecution but most of the scheduled government witnesses are police officers or people who said they heard the defendants make statements about their involvement.

Yesterday's proceedings appeared to have an invigorating effect upon prosecutors and police detectives involved in the case. At one point, Detective Ruben Sanchez-Serrano, who played a major role in the investigation, gave the thumbs-up sign.

Attorneys for some of the defendants reacted warily to the plea, with some of them predicting that Alston's plea may cause other defendants to "fall like dominoes."

Earlier attempts at plea bargaining had failed.

"We're like 10 little Indians who have just lost one," said one defense lawyer. "We're just waiting to see who is next."

Along with the 11 other defendants, Alston had been charged with kidnaping, robbery and first-degree murder -- a charge that carries a mandatory prison term of at least 20 years before the defendant becomes eligible for parole. Alston faces a maximum prison sentence of 15 years to life on the second-degree murder charge, but there is no mandatory prison term.

During yesterday's proceedings, Assistant U.S. Attorney Jerry S. Goren presented one of the most detailed accounts of the events that the prosecution alleges led up to the discovery of Fuller's battered and almost naked body in a glass-strewn garage in an alley behind the 800 block of Ninth Street NE.

Goren said the government would show that Alston and a group of people, including the 11 other defendants, were standing together in a park when they decided to rob Fuller, who was walking on the other side of the street.

Goren said the group had followed Fuller down H Street when Levy Rouse, now 20, and Charles S. Turner, now 21, grabbed Fuller from behind and pushed her into the alley. Fuller struggled and was struck by Turner and other defendants and onlookers who arrived from both ends of the alley, Goren said.

Alston, Goren said, initially stood several feet away as the lookout, but soon joined in kicking Fuller and followed the group as they dragged Fuller into the abandoned garage. In the garage, a number of the defendants continued to beat Fuller and ripped off her clothes as they searched for the coin purse she had tucked into her bra, Goren said. Then, Russell Overton, 26, and another person opened Fuller's legs while Charles Turner held her down and Rouse thrust a foot-long pipe-like object into Fuller's rectum, puncturing the rectum wall, according to Goren's account.

Alston, who told Judge Ugast he had a ninth-grade education, appeared nervous during much of the hour-long proceeding yesterday, biting his lip at times and answering "yes, sir" to a series of questions from Judge Ugast.

The plea agreement with Alston was reached about 9 Thursday night, after a lengthy meeting in Goren's office in which the prosecutor and police hammered out details of the agreement with Alston and his attorney, J. Andrew Chopivsky.

The agreement followed several days of pretrial motions in which Judge Robert M. Scott, the presiding judge in the trial, repeatedly stated that he saw no problem with admitting as evidence a videotaped statement made by Alston to police soon after his arrest. According to a transcript of that statement, Alston admitted on the tape that he served as the lookout and received some of Fuller's money, but he denied touching Fuller.

"Mr. Alston has weighed the advantages and disadvantages of going to trial," Chopivsky said after Alston's guilty plea yesterday, "and decided it was in his best interest to enter a plea of guilty to second-degree murder. Mr. Alston regrets the events of Oct. 1."

Fuller's death has sparked massive publicity due both to the number and age of the defendants -- many were teen-agers at the time -- and the severity of her beatings. A medical examiner ruled Fuller's death was caused by "blunt force injuries," including a torn liver, punctured lung, broken ribs, multiple bruises and wounds and a torn rectum wall.

After a group of about 100 prospective jurors were given a brief summary of the nature of the crime yesterday, Judge Scott asked if any felt they could not be fair and impartial. Twenty raised their hands. Jury selection resumes Monday