The aggravated indecent assault charges were filed against the 79-year-old actor after Andrea Constand, 44, a former employee at Temple University in Philadelphia, accused him of drugging her and then sexually assaulting her at his Philadelphia home in 2004.

Assistant district attorney for Montgomery County, Kristen Feden, told the court at the opening of the trial: "Trust, betrayal and the inability to consent - that's what this case is about."

Feden said Cosby previously admitted under oath that he gave Constand pills and touched her genitals as she lay on his couch. "She couldn't say no," Feden said. "She can't move, she can't talk. Completely paralyzed. Frozen. Lifeless."

A second case

Prosecutors are trying to show Cosby's treatment of Constand fit a pattern of predatory behavior. The first witness was Kelly Johnson of Atlanta who worked for one of Cosby's agents. She described an encounter she said took place at the Hotel Bel-Air in 1996. 

Johnson testified that Cosby pressured her to take a large white pill that knocked her out, and when she woke up he put lotion on her hand and forced her to touch his genitals. "My dress was pulled up from the bottom, and it was pulled down from the top, and my breasts were out," she said, crying. "And I felt naked."

The judge said only Costand and Johnson could testify to the jury, rejecting prosecution calls for 13 women who say Cosby sexually assaulted them to speak to the jury. 

An old and frail Bill Cosby was seen moving slowly into the courtroom in Norristown, Pennsylvania, leaning on a cane as well as on his assistant for support before taking a seat between his defense attorneys. Cosby had said in a recent interview that he is completely blind. His wife was absent at the courtroom.

The trial is expected to last two weeks. Cosby has said he will not testify. If found guilty, Cosby could face up to 10 years in prison as well as a $25,000 (22,000 euros) fine. He is currently on bail for $1 million (880,000 euros).

Consensual or forced relations?

Cosby's lawyers have said on numerous occasions that the encounter with Constand had been of a consensual nature - even though Constand identifies as a lesbian. Cosby has admitted in a past deposition testimony that he had touched her genitals as she lay on his couch and also that he had given her pills. The defense, however, said that the pills in question were antihistamines in this case and not prescription drugs, and that Constand had accepted them at a time when she was having difficulty sleeping.

In 2005, prosecutors had decided not to file criminal charges against Cosby after Constand had filed a civil suit which was settled the following year, according to court documents. But the criminal case against him was reopened in 2015 when several dozen women made sexual assault accusations against Cosby similar to Constand's.

In one of the disclosures in the 2005 deposition, Cosby said he had obtained Quaaludes, a known tranquilizer, in the 1970s to give to women before sex; some commentators fear that this piece of information will be used against him.

Judge Steven O'Neill stressed at the onset of the trial that the jury, comprised of seven men and five women between the ages of 20 and 80, should consider Cosby innocent until proven guilty and reminded them to refrain from glancing at news updates on their cell phones to maintain the impartiality of the trial.

Fall from grace

In 2015, details about Cosby's private life were revealed, highlighting a long history of alleged extramarital liaisons with young women. About 60 women came forward claiming he had drugged and assaulted them. However, with the cases dating back many years, the statute of limitations for prosecuting Cosby had run out in nearly every case. Andrea Constand's case is the only one to result in criminal charges against the comedian.

The trial represents a fall from grace for the African-American comedian and actor. Cosby for decades had built a good-guy image as a father and family man, both on-screen and off.

His most notable role as Dr. Cliff Huxtable on "The Cosby Show" appealed to black and white audiences alike and was celebrated as a success around the globe. His TV shows, films and comedy tours earned him an estimated $400 million. Cosby has sued the women claiming they were raped by him for defamation, a case which a federal court dismissed earlier in 2017.

The judge said he hoped to avoid drawing the kind of media attention that had surrounded O.J. Simpson's murder trial in the early 1990s. There are no cameras allowed in the Pennsylvania courtroom.

ss/jm (AP, dpa)