The trial for George Zimmerman, the Florida neighborhood watch captain accused of murdering teen Trayvon Martin while on patrol, began Monday with the judge denying the defense's request to delay the trial.

Circuit Judge Debra Nelson rejected the request after lead defense attorney Mark O'Mara told the judge his defense team needed several more weeks to prepare.

"We're not fully ready and need more time," O'Mara said.

O'Mara blamed prosecutors for a delay in turning over evidence and said he needs time to interview an attorney for Martin's family. Prosecutors opposed the request for a delay.

Jury selection in the case also started Monday.

A pool of 100 prospective jurors will be separated into groups of 21, with each prospective juror being brought into a room for questioning by the judge, state and defense.

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 Trayvon Martin.

The February 12, 2012, shooting in central Florida sparked a fierce national debate about such issues as gun control, equal justice and race after Zimmerman, who is Hispanic, fatally shot Martin, a black teenager.

There 29-year-old Zimmerman admitted to shooting Martin in the chest with his 9-mm. handgun after calling police, exiting his pickup truck to follow Martin, then getting into a fight with the 17-year-old on a rainy night inside the gated Retreat at Twin Lakes community.

But Zimmerman also says the teen circled back and attacked him as he walked back to his truck – punching him in the face and slamming the back of his head into the sidewalk.

"If there are African-Americans on the jury, they are going to sympathize with Trayvon Martin more."

- Attorney Randy McClean

Photographs taken that night show Zimmerman with a broken nose, bruises and bloody cuts on the back of his head.

Law enforcement officials at the Seminole County Courthouse Monday had been anticipating scores of protesters supporting either Martin's family or Zimmerman. But so far the crowds had stayed away on the first day of the trial. Tierrel Mathis, a Florida A&M law student, was the sole person in a fenced-off area designated for protests and she didn't even describe herself as a protester. She said she merely wanted to observe the starts to one of the highest-profile trials in central Florida.

"I thought there would be mobs of people," Mathis said.

Talking to reporters, Martin's father, Tracy Martin, expressed relief that the trial was starting.

"We seek a fair and impartial trial," Tracy Martin said. "We ask that the community continue to stay peaceful as we place our faith in the justice system."

The case is expected to last about six weeks. If convicted, Zimmerman faces life in prison.

Prosecutors are expected to argue Zimmerman racially profiled Trayvon Martin. They also are expected to say he started the fight and was a would-be police officer overstepping his authority as a watch captain, patrolling the Orlando-area community where he and the fiancée of Martin's father lived.

"The state's narrative is going to be ... Zimmerman was a powerful neighborhood watchman, a wanna-be officer who liked to use his authority," said Randy McClean, a Florida defense attorney.

Investigators know Zimmerman was on patrol following a series of break-ins inside the Sanford, Fla., community. And they know  the altercation occurred as Martin, visiting from the Miami area, was returning from a convenience store where had bought Skittles and an iced tea and that he was on a cellphone call to a female friend immediately before the altercation.

But other details and accounts of that night remain in dispute and are expected to be the focal points of the court arguments.

Prosecutors say Zimmerman tracked down Martin and started the fight.

Zimmerman says Martin spotted his gun holstered around his waist and under his clothes and said, "You are going to die tonight."

He also says Martin grabbed the gun first and fired. Martin was shot in the chest at close range and died at the scene.

Under Florida law, a person can lawfully shoot somebody in self-defense to prevent their own death or great bodily harm.

The prosecution must prove beyond a reasonable doubt that while Zimmerman's actions weren't premeditated, they demonstrated a "depraved mind" that didn't consider the threat his actions had toward human life.

O'Mara will need to show his client feared for his life and that race was not a factor in following Martin or shooting the teen, whose head was partially covered by a hoodie.  

"O'Mara's challenge is to show Trayvon wasn't profiled, that Zimmerman either saw something that looked suspicious or something else that caused him to make contact with Trayvon," McClean also said.

Zimmerman was well known to police dispatchers for his regular calls to report suspicious people and events and was told the night of the shooting to stay inside his vehicle.

Few residents have been able to give investigators a good description of what happened, and several offered conflicting accounts of who was on top of whom during the struggle.

But 911 calls made by neighbors captured cries for help during the fight and then the gunshot. Martin's parents say the cries for help were from their son, while Zimmerman's father has testified they were from his son. Voice-recognition experts could play an important role in helping jurors decide who was screaming, provided they are allowed to testify. O'Mara had raised questions about whether such prosecution experts would mislead jurors and Circuit Judge Debra Nelson has yet to rule on his concerns.

For days, the Trayvon Martin shooting received no attention beyond some small items in the local news media. Sanford police, after questioning Zimmerman, let him go and local prosecutors chose not to press charges right away.

That changed after Martin's parents hired a prominent civil rights attorney. He began complaining to the news media, accusing the police and prosecutors of letting the murderer of a black child go free, and contacting other civil rights leaders, including Jesse Jackson and Al Sharpton, to get their support.

Those events sparked protests in Sanford and around the country, with thousands demanding that Zimmerman be prosecuted. Gov. Rick Scott appointed State Attorney Angela B. Corey from the nearby Jacksonville district to re-examine the case.

Forty-four days after Martin's death, Corey charged Zimmerman with second-degree murder and had him arrested. For the past year, Zimmerman has been free on $1 million bond and living in seclusion. His defense is being paid by private contributions through a website O'Mara set up.

O'Mara has 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.

"This case, in my opinion and in my view of the facts, is a clear, straightforward self-defense case based upon the forensic evidence," he said.

Nelson has already ruled that defense attorneys won't be able to mention Martin's past marijuana use, suspension from school and past fighting during opening statements, though Nelson left open the possibility that the defense could try again later during the trial if it could show relevance.

Such a situation could arise if prosecutors attempt to portray Martin as an angelic kid.

Jurors will want to hear from Zimmerman.

"I can't see how he doesn't [testify]," said David Hill, another Florida defense attorney. "He's the only one who can say `I was scared for my life and here's what happened.' "

Another crucial witness will be Martin's female friend, who was talking with him by cellphone.

She says Martin told her during that conversation that someone was following him and that she also heard a brief exchange between him and someone before the phone was cut off.

Martin was shot shortly afterward. But O'Mara already has called into question her credibility, accusing her of lying about missing Martin's funeral because she was in the hospital.

O'Mara said he doubts he will find six jurors and four alternates who haven't heard about the case, but his goal is to find jurors who haven't formed opinions. The judge ruled that potential jurors' identities will stay anonymous in an effort to protect them from harassment and public pressure during the trial. She rejected a defense request to sequester the jury candidates during jury selection.

The defense will have a better chance with the jury if its members are older, more conservative citizens who believe in the right to bear arms, said both Hill and McClean.

"If there are African-Americans on the jury, they are going to sympathize with Trayvon Martin more," McClean said.

Prosecutors have refused to comment about the case outside the courtroom, but lead prosecutor Bernie de la Rionda indicated at a recent hearing he was well aware of the pressures the case's high profile is putting on all parties involved. Reporters from national media groups are attending the trial, and areas outside the courthouse have been designated for expected protests.

"We want to make sure this trial is tried in a courtroom and not outside a courtroom," de la Rionda said.

The Associated Press contributed to this report.