The first group of 100 potential jurors filled out questionnaires about themselves and their ability to serve on the jury as prosecutors and defense attorneys sought to find six objective members and four alternates. In Florida, 12 jurors are required only for criminal trials involving capital cases, when the death penalty is being considered.
Zimmerman was present in the jury holding room as his defense attorneys and prosecutors introduced themselves to the potential jurors.
Attorneys from both sides then leafed through the questionnaire responses in the courtroom.
The selection of jurors who both the prosecution and defense believe can be objective in the highly publicized case is expected to take all week, if not longer. The judge has said she will keep the identities of the selected jurors anonymous but she rejected a defense request to sequester the entire jury pool of 500 residents.
Prosecutors and defense attorneys asked prospective jurors what they had heard about the case on television, in newspapers and on the Internet. A woman in her 50s who watches TV games shows, said she believed she could be unbiased even though she knew some basic facts of the case. Another woman in her late 30s who recently moved from Chicago and works in a nursing home, said she only had a passing familiarity with the case - mainly images she saw of people wearing T-shirts with Martin's face on them.
"I really don't know anything about the case," said the woman, known as Juror B29. "But I believe at the end of the day, you have to listen to both sides."
Judge Debra Nelson said the jury selection would alternate with the continuation of a hearing to determine whether she will allow the testimony of voice-recognition experts who say they might be able to identify who was screaming on a 911 tape recorded during Zimmerman's confrontation with Martin.
Thus far, the experts have reached mixed conclusions. Defense attorneys don't want them to testify.
In her first order of business, Nelson denied a request by defense attorney Mark O'Mara to delay the trial because he needed several more weeks to prepare.
O'Mara blamed prosecutors for a delay in turning over evidence and said he needed time to interview an attorney for Martin's family. "We're not fully ready and need more time," O'Mara said.
Prosecutors opposed the request for a delay.
The trial was taking place in the Orlando suburb of Sanford, Fla., the scene of massive protests last year by people who were angered that police waited 44 days before charging Zimmerman. Other demonstrations were held around the country, and the case drew worldwide attention as it fanned a debate about race, equal justice under the law and gun control.
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.