Bill Cosby went on trial Monday on charges he drugged and sexually assaulted a woman more than a decade ago, with prosecutors immediately introducing evidence the 79-year-old TV star once known as America's Dad had done it before to someone else.

The prosecution's opening witness was not the person Cosby is charged with abusing, but another woman, who broke down in tears as she testified that the comedian violated her in the mid-90s at a hotel bungalow in Los Angeles.

Cosby is on trial on charges he assaulted Toronto's Andrea Constand, a former employee of Temple University's basketball program, at his suburban Philadelphia mansion in 2004. His good-guy reputation already in ruins, he could get 10 years in prison if convicted.

In her opening statement, prosecutor Kristen Feden noted that the Cosby Show star 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."

Cosby attorney Brian McMonagle countered by attacking what he said were inconsistencies in Constand's story, disputed that Constand was incapacitated and made the case that she and Cosby had a romantic relationship.

He said Constand initially told police that she and Cosby did not speak after their 2004 encounter, when, in fact, phone records show the two talked 72 times, with 53 of those calls initiated by Constand.

'It was just me, just my word against his'

Constand, 44, is expected to take the stand this week and tell her story in public for the first time.

The trial's first witness was Kelly Johnson of Atlanta, who worked for one of Cosby's agents at the William Morris Agency. She described an encounter she said took place in 1996 at the Hotel Bel-Air when she was in her mid-30s.

Andrea Constand is shown in Toronto in 2015. (Marta Iwanek/The Canadian Press)

Prosecutors are trying to show Cosby's treatment of Constand fit a pattern of predatory behavior. They had wanted to call as many as 13 women who say Cosby sexually assaulted them, out of more than 60 accusers in all. But Judge Steven O'Neill, in a victory for Cosby, said the jury could hear only from Constand and Johnson.

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." Cosby's lawyer argued that Johnson was seeking a payout from the TV star.

Attorney Gloria Allred climbs the stairs inside the Montgomery County Courthouse at Bill Cosby's sexual assault trial. (David Maialetti/The Philadelphia Inquirer via Associated Press)

McMonagle said Johnson mixed up the years and other details of her encounters with Cosby, and he grilled her about why she never said anything when she left William Morris. She came forward in 2015 at a news conference with celebrity lawyer Gloria Allred.

"I felt embarrassed because I had a secret about the biggest celebrity in the world at the time and it was just me, just my word against his, and I was very afraid," Johnson said.

Cosby grinned and tapped his wooden cane as his lawyer questioned Johnson.

Dozens of women say he drugged, assaulted them

The comedian arrived at the courthouse in the morning holding his spokesman's arm for support as he walked past dozens of cameras.

Cosby's wife, Camille, was not in court. But actress Keshia Knight Pulliam, who played his daughter Rudy on The Cosby Show in the 80s and 90s, was at his side as he made his way into the building. She told reporters she was there to support her TV dad.

"I want to be the person that I would like to have if the tables were turned," she said. "Right now it's the jury's job and the jury's decision to determine guilt or innocence. It's not mine or anyone else's."​

Cosby built a good-guy reputation as a father and family man, on screen and off, during his 50-year career in entertainment. He created TV characters, most notably Dr. Cliff Huxtable, with crossover appeal among blacks and whites alike. His TV shows, films and comedy tours earned him an estimated $400 million.

Bill Cosby arrives for his sexual assault trial at the Montgomery County Courthouse in Norristown, Pa., on Monday. (Matt Rourke/Associated Press)

Then a deposition unsealed in 2015 in a lawsuit brought by Constand revealed an unsavoury private life marked by a long history of extramarital liaisons with young women. Dozens of women came forward to say he had drugged and assaulted them.

Those developments led prosecutors in Pennsylvania to bring charges against Cosby a decade after the district attorney at the time concluded the case was too weak.

Cosby's lawyers tried to get case thrown out​

The statute of limitations for prosecuting Cosby had run out in nearly every case. This is the only one to result in criminal charges against the comic.

Feden, the prosecutor, warned the jury not to fall into the trap of confusing celebrities with the characters they play. "We think we really know them," she said. "In reality, we only have a glimpse of who they really are."

Cosby's lawyers tried repeatedly to get the case thrown out, arguing that a previous district attorney promised him he would never be charged, and that witnesses have died, memories have faded and the comedian is all but blind.

The AP does not typically identify people who say they are sexual assault victims unless they grant permission, which Constand and Johnson have done.