A former security guard at Michael Jackson's Neverland ranch has testified that he saw the singer sexually abusing a boy in the early 1990s.
Jackson denies any wrongdoing
Ralph Chacon gave graphic evidence at the trial in California. But defence lawyers attacked his credibility, painting him as a bitter ex-employee.
A former maid also alleged she had seen Mr Jackson kissing three boys, as well as the actor Macaulay Culkin.
Mr Jackson, 46, denies a total of 10 charges including child abuse.
Defence lawyers painted both witnesses as disgruntled employees who had both unsuccessfully sued Mr Jackson for wrongful dismissal.
In what correspondents describe as damaging testimony, former guard Mr Chacon said he first saw Mr Jackson and the boy - who was then 10 - in a whirlpool bath.
The two then headed to the shower together, Mr Chacon said.
He added that he decided to look through an open window to see what was happening and saw Mr Jackson performing a sex act on the boy.
Ralph Chacon unsuccessfully sued Mr Jackson
"I was thinking: 'What's going on in there?' Grown man in the shower with a boy... it wasn't right," he told the court in the town of Santa Maria. "I saw Mr Jackson caressing the boy's hair, he was kissing him."
Mr Chacon, who first mentioned the alleged assaults when being questioned under oath in 1994, said he did not report the incident to police as he did not think he would be believed.
The boy involved in the alleged assault later won an out-of-court settlement from the pop star reportedly worth more than $20m (£10.7m).
During cross-examination, Mr Jackson's lawyer Tom Mesereau accused Mr Chacon of trying to extort the star.
He said Mr Chacon, who worked for Mr Jackson between 1991 and 1994, had become bitter after losing a lawsuit against Mr Jackson for wrongful dismissal.
Mr Mesereau said Mr Jackson had countersued and Mr Chacon was ordered to pay more than $1m in legal costs and damages, as well as $25,000 for stolen property.
Former maid Adrian McManus testified on Thursday that Mr Jackson had routinely shared his bed with three boys
She said she was also concerned by Mr Jackson's behaviour toward the actor Macaulay Culkin.
"I saw Mr Jackson and Macaulay by the library," she said.
"Mr Jackson was kissing him on his cheek, and he had his hand kind of on his leg, kind of on his rear end."
The defence asserted that Mr Culkin has repeatedly stated he was never molested
Ms McManus said she had also once witnessed the singer touching the genitals of the same boy Mr Chacon said he had seen showering with Mr Jackson.
Adrian McManus was a former maid at Neverland
Under cross-examination she said she had lied in a 1993 court deposition when she reported she had not seen any inappropriate behaviour by Mr Jackson at Neverland. She claimed Mr Jackson threatened to report her to her superiors.
Defence lawyer Thomas Mesereau pointed out Ms McManus was part of the lawsuit filed against Mr Jackson for unlawful dismissal, which was thrown out for being fraudulent and malicious.
Ms McManus is expected to take the stand again on Friday to be questioned further by the defence.