Wayne B. Williams' attorneys launched their defense today by chipping away at the credibility of prosecution witnesses, including a woman who testified that she saw one victim, Larry Rogers, with the accused killer the day he disappeared.

That witness, Nellie Trammell, frequently dropped by police task force headquarters on Peachtree Street to relate her latest dreams or "visions" about the 28 baffling murders of young blacks to investigators working the cases, said Kenneth Lawson, who worked at the headquarters.

Lawson, 31, a former recruit who resigned from the force last June to avoid being fired, portrayed the woman as a "psychic" who often stayed around to chat with officers and do her knitting, one among an army of cranks who nettled the officers.

"Every time a body was found, she would come to the task force and relate her visions," Lawson said.

Trammell testified last week that she saw Rogers slumped over in the front seat of Williams' station wagon on March 30, 1981. Williams was behind the wheel, she said. Rogers is among the 10 slaying victims the prosecution has been allowed to introduce to show a pattern applying to those killings and the two with which Williams is charged. He is accused of killing Jimmy Ray Payne, 21, and Nathaniel Cater, 27.

The defense also attacked the credibility of two police rookies who were on bridge stakeout duty the night Williams was stopped for questioning. Williams was grilled by police May 22 when he drove slowly across the Jackson Parkway Bridge shortly after a recruit heard a splash in the Chattahoochee River below.

Another recruit, Freddie Jacobs, testified that he saw Williams' car creeping slowly across the bridge seconds after the splash. Cater's body washed up downstream two days later.

But Lawson, who worked stakeout at another bridge, portrayed his fellow recruits as Keystone cops-in-training who drank on duty and took turns sleeping in a pup tent. He said Jacobs was so afraid of the dark that he needed an escort to relieve himself in the woods.

The night before Williams was stopped, Lawson said, Jacobs reported a "hazy, white figure" throwing a body off the bridge. SWAT officers investigated and concluded that "Freddie had seen a ghost," Lawson said.

The stakeouts were supposed to be top secret, but police turf squabbles and officers dressed out like campers attracted the attention of nearby housing project residents, who turned out with shouts of "Here comes a body" as they tossed garbage off the bridge, he said.

Lawson acknowledged that, had he not resigned from the police department, he would have been fired for poor grades at the training academy, missing four days of training, and failing to report a minor domestic incident for which he was detained by police.

Another defense witness, a pediatric pathologist who reviewed autopsy reports on 12 victims, contradicted local medical examiners who concluded both Cater and Payne were murdered. Dan Stowens, lab director at St. Luke's University Hospital in Ithaca, N.Y., an authority on sudden infant death syndrome, said he believed Payne drowned.

There was not enough evidence to rule Cater's death a homicide, he said, basing his opinion on a review of autopsy reports and some tissue slides.

He also disputed medical examiners finding that Cater had been in the water two to five days. He said Cater's body was so decomposed, it was likely he had been dead since May 17.

Two eyewitnesses placed Cater alive on May 21, one of them with Williams. Asked about the discrepancy, he said, "Either eyewitnesses are mistaken or this body is not Nathaniel Cater." Authorities identified Cater from fingerprints.