NORRISTOWN, Pa. — Jurors in the sexual assault trial of Bill Cosby were presented Monday with two fundamentally  contrasting portraits of the comic legend.

In a searing opening statement, a Pennsylvania prosecutor, her voice filled with scorn, portrayed Cosby as a menacing figure who used his fame and power to take advantage of a woman he’d been mentoring.

“This case is about trust, betrayal and the inability to consent,” prosecutor Kristen Feden said with a look of contempt on her face.

Cosby’s defense attorney countered by turning attention to the entertainer’s accuser, describing the alleged victim as a schemer motivated by greed and a serial liar.

“The only thing that is worse than [sexual assault] is the false accusation of sexual assault,” Cosby’s attorney, Brian McMonagle, said in a booming voice. “It’s an attack on human dignity.”

Cosby is accused of drugging and sexually assaulting Andrea Constand, an operations manager for the Temple University women’s basketball team at his suburban Philadelphia home in 2004.

Both sides gave jurors a preview of their approaches to the case, with prosecutors focusing heavily on the pills that Cosby has admitted giving Constand and the defense blasting away at inconsistencies in Constand’s statements to police.

Feden, who stepped into a starring role after serving as more of a side player in pre-trial hearings, repeatedly stalked across the floor, pointing an accusatory finger within inches of Cosby as he sat leaning forward at the defense table. When Feden said that Cosby drugged Constand because he wanted to “gratify himself” without any chance of rejection, the entertainer balled his left hand into a fist and held it to his mouth.

Feden, who is African American, sought to separate the image of Cosby as a lovable, trailblazing African American entertainer from the more sinister accusations at the heart of the case. She implored jurors to not think of Cosby as Dr. Huxtable, the character he portrayed on television in “a role that is able to transcend race, transcend age and any barrier.”

The nuances of race have loomed over the case. Cosby’s daughter, Ensa, said before jury selection began that her father is a victim of a “public lynching.” Cosby’s attorneys accused prosecutors of trying to block African American jurors from being seated, an argument the judge rejected.

Feden, for the first time, revealed Monday that prosecutors plan to introduce evidence from a toxicologist who will testify that Constand’s symptoms on the night of the alleged assault are consistent with the effects of Quaaludes. Cosby has said that he acquired Quaaludes to give to women with whom he wanted to have sex; his representatives have said the drugs were for consensual use and were only used during the wild 1970s.

In police interviews, Cosby has said that he gave Benadryl to Constand, describing the pills to her as herbal supplements that he referred to as “friends to help you relax.” Anticipating that defense, Feden told jurors that Cosby has also said that Benadryl could induce drowsiness. Cosby told police investigators that he takes Benadryl as a sleep aid when he’s on tour and that he wouldn’t appear on stage after ingesting the allergy medication.

“He knew what effect it would take,” Feden said.

Feden’s scorn was matched in intensity by McMonagle, Cosby’s charismatic attorney. A former prosecutor who is now one of Philadelphia’s most prominent defense attorneys, McGonagle railed against Feden’s suggestion that the defense would be built around an effort to distract jurors from the main facts of the case.

McMonagle, alternating between an intimate whisper and a window-rattling voice of doom, issued a scathing assessment of Constand, saying that she had been “untruthful time and time and time again.” He pleaded with jurors to bore down into the details.

“This isn’t some movie!” he said.

The biggest flaws in the prosecution case, according to McMonagle, are the various statements Constand, who was 30 at the time of the alleged incident, made to police. In her first police interviews, she said that she’d never been alone with Cosby before the night of the alleged assault and that she had no contact with him afterward. Later, according to McMonagle, she told investigators that she’d been alone with him several times and had brought him gifts, such as bath salts and incense. And phone records, McMonagle said, show that Constand called Cosby 53 times after the alleged incident.

Even the circumstances on the night of the alleged assault have become muddied in Constand’s statements, McMonagle said. Initially she told police that Cosby assaulted her after a dinner out at a restaurant with a group of people. Later, McMonagle said, she changed her story and said that the assault took place when Cosby and Constand were alone together at the entertainer’s home.

Cosby entered the courtroom leaning on the arm of Keshia Knight Pulliam, an actress who long ago played his character’s youngest child, Rudy, on “The Cosby Show.”

Cosby wore a dark blue suit and a tie with slanting purple and yellow stripes. As the courtroom filled, the entertainer was animated and chatty, laughing and cracking jokes with his attorneys at the defense table. Later, as Judge Steven T. O’Neill instructed the jury, Cosby sat looking directly across the room at the seven men and five women who will decide whether he is guilty of three counts of aggravated indecent assault. As he listened, Cosby, who says he is legally blind, cupped his hands on the handle of the slender wooden cane he has carried throughout months of pretrial hearings.

A few steps behind Cosby, and to his left, one of his primary pursuers, the feminist attorney Gloria Allred, took an aisle seat in the second row of the public seating area. Allred represents more than half of the 60 women who have publicly said that Cosby committed sexual crimes against them over five decades. Among the Allred clients is a woman who goes by the pseudonym “Kacey” — the sole accuser, other than Constand, who will be allowed to testify for the prosecution. Three rows behind Allred sat Joe Cammarata, a District-based attorney who represents seven accusers in a separate defamation suit against Cosby.

The sequestered jury has been bused across the state of Pennsylvania from Pittsburgh, 300 miles away. They were selected from a different county because of defense concerns about pretrial publicity in Montgomery County, Pa., where the district attorney who is trying the case, Kevin Steele, pledged to reopen the Cosby investigation during his 2015 election campaign.

Most of the jurors dressed casually for the opening day of the trial, with several young men wearing blue jeans. The lone senior citizen on the juror arrived in a white jacket and carrying a cane.

The courthouse where the trial is being held in Norristown, Pa., a working-class city outside Philadelphia far from the swanky Main Line suburbs, is a large columned structure with a cupola and a set of steep stairs ascending to the lobby. But it’s showing its age: O’Neill apologized to jurors for the cramped seating, saying the courtroom is “antiquated.” Still, the courtroom has a certain grandeur, with tall ceilings, dark-stained wood paneling and royal purple carpeting. Eight massive chandeliers loom overhead, and an enormous Colonial-era painting hangs behind the judge’s bench.

Three of Cosby’s accusers who will not be testifying — Victoria Valentino, Therese Serignese and Lili Bernard — arrived at the courthouse hours early. They had hoped to sit in the same room as Cosby, offering a silent form of protest against the man they say drugged and sexually assaulted them. But they couldn’t get close. There was no room for them in the courtroom, and the women were ushered to a separate room with a video feed instead.

This report has been updated.