Judge Clarence Cooper today agreed to let prosecutors introduce evidence linking Wayne B. Williams to the slayings of 10 young blacks in addition to the two with which he has been charged.

Defense attorney Alvin Binder immediately asked for a mistrial based on the decision, but Cooper denied the motion.

It was a major victory for the prosecution in their case against Williams, 23, who is accused of killing Nathaniel Cater, 27, and Jimmy Ray Payne, 21, two of 28 victims on a task force list of young blacks who were found murdered here in a two-year period.

The jury today began hearing the first gruesome details of the trail of bodies that has baffled police and the special task force set up to investigate the slayings.

All except one of the 10 additional victims also comes from the task force list.

Cooper ruled that evidence from the 10 cases may be used only in an attempt to prove that the killings of Cater and Payne were part of an ongoing pattern.

Prosecutors revealed Friday that investigators have fibers and bloodstain evidence tying Williams to the 10 killings, as well as witnesses who can place him with three of the victims. Fiber evidence from the 10 victims is expected to bolster last week's testimony from an FBI chemist who matched material from Williams' home and car with fibers and dog hairs found on Cater and Payne. The matched fibers, he said, meant that the victims had come into contact with Williams.

Prosecutors contend Williams also came into contact with Alfred James Evans, 13, whose decomposing body was found July 28, 1979. Atlanta police detective Mickey Lloyd said Evans' parents looked at photographs of the body, but refused to believe it was their son. The body lay in the morgue for more than a year until misfiled dental charts confirmed his identity.

Binder asked Lloyd on cross-examination if he thought he was better able to identify the boy than his mother had been.

"Being an investigator, and lacking the emotions of the mother, I think I could," said Lloyd.

Prosecutors spent the day introducing details surrounding three of the 10 victims: Evans, Charles Stephens, 12, and Lubie Geter, 13. All died from either strangulation or asphyxiation, medical examiners testified.

The Rev. Burris T. Gibbs, grandfather of Charles Stephens, was asked whether he had identified his grandson at the morgue. "I don't want to remember that day," he replied.

"Oh, God," he said, and shuddered, when an assistant district attorney asked him to identify his grandson's photograph from the morgue.

Coroners testified that Stephens had been asphyxiated and Evans and Geter were probably strangled. Dr. John Feegel, an assistant county medical examiner, admitted under cross-examination that he hadn't found any marks on Evans' neck. But, he said, "you could accomplish death by strangulation without leaving any marks." Authorities have ruled that Cater was strangled and Payne asphyxiated.

Feegel also reminded jurors that no similar killings have occurred since Williams' arrest. "Right now, we're in an unusual situation," Feegel said. "We have not had . . . any children's homicides in many months."

Fibers were found in Stephens' hair and on his chest and stomach, testified an East Point police detective who sealed off the crime scene, but no attempt was made today to tie those fibers to any found in Williams' home or car.