Disgraced US megastar Bill Cosby went on trial for sexual assault in Pennsylvania on Monday, accused of drugging and sexually assaulting a university employee at his Philadelphia mansion 13 years ago.

The 79-year-old actor and comedian who for decades was feted by millions as "America's Dad" -- and risks spending the rest of his days behind bars if convicted -- sat silently as the prosecution painted a picture of him as a sexual predator.

In what is one of America's biggest celebrity trials in years, he faces three counts of aggravated indecent assault, which each carry a maximum sentence of 10 years in jail and a $25,000 fine.

Around 60 women have publicly accused the pioneering black comedian of being a serial sexual predator for decades, shredding his reputation, ending his career and leaving him isolated by friends and celebrities.

But the trial concerns the allegations of one woman, Andrea Constand, over one incident in January 2004. It is the only criminal case against Cosby as the vast majority of alleged abuse happened too long ago to prosecute.

Constand, a 44-year-old Canadian, was at the time director of basketball operations at Temple University, where Cosby served on the Board of Trustees until his resignation in a blaze of scandal in 2014.

In a 45-minute opening statement, the prosecution said the star of the hit television series "The Cosby Show" exploited his power and celebrity to abuse the trust of a woman who considered him a friend and mentor.

'Paralyzed'

Constand alleges Cosby plied her with pills and wine before sexually assaulting her on his couch after she had gone to his house to seek career advice on moving to Canada and studying massage therapy.

"As she went in and out of consciousness, she witnessed her body being used to sexually gratify the defendant," Montgomery County assistant district attorney Kristen Feden said. "Because she was in that incapacitated state, she couldn't consent."

"Completely paralyzed, frozen, lifeless," the prosecutor said of Constand.

Feden said the evidence would "shatter" illusions that Cosby was the real-life embodiment of his signature character, lovable obstetrician and benevolent father Cliff Huxtable, a role that smashed through racial barriers.

Cosby, who says he is now legally blind, arrived at the courthouse in the Philadelphia suburb of Norristown accompanied by Keshia Knight Pulliam, who played his daughter Rudy on the 1984-1992 sitcom.

His wife of 53 years, Camille, who has stood by him, has attended none of his pre-trial hearings, and the majority of his celebrity friends have deserted the award-winning star.

Cosby says he gave Constand Benadryl to relieve stress, insists their sexual relations were consensual and accuses her of lying.

The defense used its opening statement to lecture the jury on the high burden of proof in a criminal case and warned them not to get distracted by his fame or by "finger-pointing" from the prosecution.

"The false accusation of sexual assault -- it's an attack on human dignity, it's an attack on human innocence. It's not a distraction, it can destroy a man," said defense lawyer Brian McMonagle.

'Just a citizen'

McMonagle urged the jury to see Cosby as "just a citizen," saying his infidelity as a husband had made him "vulnerable to accusations" and that while he had achieved greatness, he had also endured tragedy.

Constand, who is gay, waited a year to report the incident and initially settled the case with a civil suit in 2006.

The case was re-opened in 2015 when Montgomery County claimed new evidence had come to light.

But securing a conviction or an acquittal will be fraught with difficulties -- and it ultimately comes down to her word against his.

The prosecution will put Constand on the stand and seek to prove a pattern of behavior in also calling a former personal assistant to Cosby's agent, who alleges that he assaulted her in a similar manner in 1996.

They also intend to bring a toxicologist to testify about the effects of the pill and make use of Cosby's words in a 2005 court deposition, in which he admitted obtaining sedatives with a view to having sex.

The defense on Monday tore into Constand's credibility in its opening statement, alleging that she was untruthful "time and time again" and painted their relationship as consensual, one that involved many meetings and alcohol, and alleged that Constand telephoned the star 53 times.

The jury, selected 300 miles (480 kilometers) away in Pittsburgh, will be sequestered for the duration of the trial in a hotel.

While Cosby has said he does not expect to testify, his lawyers have not ruled out the possibility.