A key prosecution witness testified yesterday that he and nine of the 10 young people on trial for the slaying of Catherine L. Fuller repeatedly punched and kicked Fuller, but said he was unable to explain why a random robbery over a coin purse turned into one of the most brutal killings in recent District history.

In a trembling voice, Calvin L. Alston, 20, the second man to plead guilty in Fuller's slaying, described to a D.C. Superior Court jury how a spur-of-the-moment robbery on a misty afternoon suddenly turned into a scene of mass "pushing and shoving" as a group of about a dozen young people, friends, tried "to get their licks in" with Fuller.

Alston, breaking into tears, told the jury that Fuller was not his first choice for a robbery, but that he had pointed her out after someone in the group vetoed his suggestion that they rob a bus passenger.

But when asked under extensive cross-examination why he struck and kicked Fuller when it was obvious that she was already overcome by the others' beatings, Alston hesitated before giving a series of confusing answers.

Alston first said "yes" when defense lawyer Frederick Sullivan asked if he had begun to attack Fuller just for the sake of attacking her, but then later said "no" when Sullivan asked if he didn't "just want to hurt" Fuller, 48, whose 99-pound, battered body was found Oct. 1, 1984, in an abandoned Northeast garage.

"I was just throwing punches," Alston answered quietly.

Alston, who wore a light gray suit as he testified on the the sixth day of the trial of 10 young people accused of slaying Fuller, was the second prosecution witness to identify most of the defendants as participants in Fuller's robbery or death. Alston failed yesterday to name only one of the accused -- 17-year-old Felicia Ruffin, the only woman among the defendants.

Several defense lawyers yesterday described Alston's testimony as the most damaging so far in the trial before Judge Robert M. Scott, and said privately they feared Alston's statements might severely harm their cases. Most notably, several lawyers said they believed Alston's testimony strengthened the eyewitness account of Harry James Bennett, who testified earlier this week.

Bennett, who was the prosecution's key witness until Alston decided to plead guilty two weeks ago, also identified nine of the defendants as participants, including Ruffin. The defendant he did not place at the scene is 17-year-old Clifton Yarborough.

Alston did implicate Yarborough yesterday, saying Yarborough asked him to serve as a lookout while the crime was being committed and later gave him $10 for "his work."

There were differences between Bennett's and Alston's accounts as to when certain people participated. However, prosecutors said they were pleased with Alston's testimony yesterday and they are expected to argue later that the two men's accounts were essentially similar.

Speaking clearly and more concisely than Bennett during his direct examination by Assistant U.S. Attorney Jerry S. Goren, Alston yesterday spent much of his time on the stand describing in graphic detail how the group of young people, gathered in their regular meeting spot at a park and talking about "getting paid," started out to rob Fuller but ended up killing her.

Alston at one point pulled his arm back over his head to illustrate his account of how defendant Levy Rouse, 20, smacked Fuller on the head with a "two-by-four" after she first struggled with her assailants.

Later, in one of the most emotional moments of the trial, Alston left the witness stand and crouched down before the jury to show how, according to his account, Rouse thrust a pole into Fuller's rectum after she was dragged into the garage.

After telling lawyer Sullivan that he could not remember what Fuller was carrying the day he decided to rob her or if anyone took her jewelry, Alston responded that he had not "lost sight of the objective" to rob Fuller when the beating began, nor did he simply want to hurt her.

What was Mrs. Fuller doing then to prompt the attack? Sullivan asked. "Was she insulting you . . . was she trying to crawl away . . . was she crying?"

"I don't remember," answered Alston.