There is no dispute Zimmerman shot an unarmed Martin, 17, during a fight on a rainy night in February 2012. Prosecutors will try to show the neighborhood watch volunteer racially profiled the black teenager, while Zimmerman's attorney must convince jurors Zimmerman pulled his 9 mm handgun and fired a bullet into the high school student's chest because he feared for his life.
Zimmerman, who identifies himself as Hispanic, says he shot Martin in self-defense. If convicted, he could get a life sentence.
Under Florida's "stand your ground" law, Zimmerman, 29, could shoot Martin in self-defense if it was necessary to prevent death or great bodily harm. O'Mara previously decided not to invoke a "stand your ground" hearing in which a judge alone would decide whether to dismiss the case or allow it to proceed to trial.
Martin's father, Tracy Martin, expressed relief that the trial was starting.
"We seek a fair and impartial trial," he told reporters Monday. "We ask that the community continue to stay peaceful as we place our faith in the justice system."
Zimmerman's brother, Robert Zimmerman Jr., said his family was confident that prosecutors wouldn't meet the burden of proving the neighborhood watch volunteer was guilty of murder. He called the charge "improper" and said charges were filed for political reasons.
"You don't charge in this country simply to assuage the concerns of masses," said Robert Zimmerman Jr. "Unfortunately, a political calculation was made centered around the politics of race and the law was defiled."
Law enforcement officials at the Seminole County Courthouse had been anticipating scores of protesters supporting either Martin's family or Zimmerman. But the crowds stayed away on the first day of the trial, with just a little more than a dozen protesters showing up. They included relatives of Oscar Grant, a man who was fatally shot by an Oakland, Calif., police officer in 2009, and members of a Communist Party group.
Cephus Johnson, Grant's uncle who flew in from California, said he wanted to stand with Martin's family at the start of the trial.
"We know how important it is to be embraced by families who have gone through the same things," Johnson said.
Wearing a hoodie similar to the one Martin had on when he was shot, Noche Diaz, a leader of the Revolutionary Communist Party, and about 10 other party activists carried placards that said "We Say No More." Diaz said it was important for protesters to have a presence at the trial's start since "if people waited in the first place, there wouldn't be a trial."
Tierrel Mathis, a Florida A&M law student who wanted to observe the start to one of the highest-profile trials in central Florida, was surprised there weren't more protesters."I thought there would be mobs of people," Mathis said.