Neighbourhood watch volunteer George Zimmerman was cleared of all charges Saturday in the shooting of Trayvon Martin, the unarmed black teenager whose killing unleashed furious debate across the U.S. over racial profiling, self-defence and equal justice.

Zimmerman, 29, blinked and barely smiled when the verdict was announced. He could have been convicted of second-degree murder or manslaughter. But the jury of six women, all but one of them white, reached a verdict of not guilty.

Martin's mother and father were not in the courtroom when the verdict was read; supporters of his family who had gathered outside yelled "No! No!" upon learning of the not-guilty verdict.

The teen's father, Tracy, reacted on Twitter: "Even though I am broken hearted my faith is unshattered I WILL ALWAYS LOVE MY BABY TRAY."

'Et tu America'

Trayvon Martin's brother, Jahvaris Fulton, said simply: "Et tu America?" — a reference to the Latin phrase "Et tu, Brute?" known as an expression of betrayal.

The jurors considered nearly three weeks of often wildly conflicting testimony over who was the aggressor on the rainy night the 17-year-old was shot while walking through the gated townhouse community where he was staying and where Zimmerman lived. None of the witnesses who were called had a clear view of their encounter.

Defence attorneys said the case was classic self-defence, claiming Martin knocked Zimmerman down and was slamming the older man's head against the concrete sidewalk when Zimmerman fired his gun.

"We're ecstatic with the results," defence lawyer Mark O'Mara after the verdict. "George Zimmerman was never guilty of anything except protecting himself in self-defence."

Another member of his defence team, Don West, said he was "thrilled this jury kept this tragedy from becoming a travesty."

Darrsie Jackson reacts after hearing the verdict of not guilty in the trial of George Zimmerman with her children at the Seminole County Courthouse. (John Raoux/Associated Press)

Prosecutors called Zimmerman a liar and portrayed him as a "wannabe cop" vigilante who had grown frustrated by break-ins in his neighbourhood committed primarily by young black men. Zimmerman assumed Martin was up to no good and took the law into his own hands, prosecutors said.

State Attorney Angela Corey said after the verdict that she believed second-degree murder was the appropriate charge because Zimmerman's mindset "fit the bill of second-degree murder."

"We charged what we believed we could prove," Corey said.

The jurors' names have not been made public, and they declined to speak to the news media.

As the verdict drew near, police and city leaders in the Orlando suburb of Sanford and other parts of Florida said they were taking precautions against the possibility of mass protests or unrest in the event of an acquittal.

Hanna Brianna, 5, holds a sign in front of her home in Sanford, Fla. (Mike Brown/Associated Press)

Martin family lawyer Benjamin Crump acknowledged the disappointment of Trayvon Martin's supporters, ranking the teen alongside civil rights heroes Medgar Evers and Emmett Till in the history of the fight for equal justice.

But he urged them not to resort to violence.

"For Trayvon to rest in peace, we must all be peaceful," he said.

O'Mara, Zimmerman's lawyer, said his client is aware he has to be cautious and protective of his safety.

"There still is a fringe element that wants revenge," O'Mara said. "They won't listen to a verdict of not guilty."

The verdict came a year and a half after civil rights protesters angrily demanded Zimmerman be prosecuted. That anger appeared to return Saturday night outside the courthouse, at least for some who had been following the case.

Rosie Barron, 50, and Andrew Perkins, 55, both black residents of Sanford, stood in the parking lot of the courthouse and wept.

Patrick Woodburn, left, and William Memola hold signs supporting George Zimmerman in front of the Seminole County Courthouse. (Phelan M. Ebenhack/Associated Press)

"I at least thought he was going to get something, something," Barron said.

Added her brother: "How the hell did they find him not guilty?"

Perkins was so upset he was shaking. "He killed somebody and got away with murder," Perkins shouted, looking in the direction of the courthouse. "He ain't getting no probation or nothing."

Issuing a statement from New York, civil rights leader the Rev. Al Sharpton called the not-guilty verdict "a slap in the face to the American people but it is only the first round in the pursuit of justice."

Rev. Sharpton heading to Florida

"We intend to ask the Department of Justice to move forward as they did in the Rodney King case and we will closely monitor the civil case against Mr. Zimmerman," Sharpton said, adding that he will travel to Florida in the next few days.

Several Zimmerman supporters also were outside the courthouse, including a brother and sister quietly rejoicing that Zimmerman was acquitted. Both thought the jury made the right decision in finding Zimmerman not guilty — they felt that Zimmerman killed Martin in self-defence.

Cindy Lenzen, 50, of Casslebury, and her brother, 52-year-old Chris Bay, stood watching the protesters chant slogans such as, "the whole system's guilty."

Lenzen and Bay — who are white — called the entire case "a tragedy," especially for Zimmerman.

"It's a tragedy that he's going to suffer for the rest of his life," Bay said. "No one wins either way. This is going to be a recurring nightmare in his mind every night."

Meanwhile, authorities in Martin's hometown of Miami said the streets were quiet, with no indication of problems. The neighborhood where Martin's father lives in Miami Gardens was equally quiet.

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Zimmerman wasn't arrested for 44 days after the Feb. 26, 2012, shooting as police in Sanford insisted that Florida's Stand Your Ground law on self-defence prohibited them from bringing charges. Florida gives people wide latitude to use deadly force if they fear death or bodily harm.

Martin's parents, along with civil rights leaders such as Sharpton and the Rev. Jesse Jackson, argued that Zimmerman — who identifies himself as Hispanic — had racially profiled their son. And they accused investigators of dragging their feet because Martin was a black teenager.

Before a special prosecutor assigned to the case ordered Zimmerman's arrest, thousands of protesters gathered in Sanford, Miami, New York and elsewhere, many wearing hoodies like the one Martin had on the night he died. They also carried Skittles and a can of iced tea, items Martin had in his pocket. U.S. President Barack Obama weighed in, saying that if he had a son, "he'd look like Trayvon."

Despite the racially charged nature of the case, race was barely mentioned at the trial.

"This case has never been about race or the right to bear arms," Corey said. "We believe this case all along was about boundaries, and George Zimmerman exceeded those boundaries."

One of the few mentions of race came from witness Rachel Jeantel, the Miami teen who was talking to Martin by phone moments before he was shot. She testified that he described being followed by a "creepy-ass cracker" as he walked through the neighbourhood.

Jeantel gave some of the trial's most riveting testimony. She said she overheard Martin demand, "What are you following me for?" and then yell, "Get off! Get off!" before his cellphone went dead.

The jurors had to sort out clashing testimony from 56 witnesses in all, including police, neighbours, friends and family members.

For example, witnesses who got fleeting glimpses of the fight in the darkness gave differing accounts of who was on top. And Martin's parents and Zimmerman's parents both claimed that the person heard screaming for help in the background of a neighbor's 911 call was their son. Numerous other relatives and friends weighed in, too, as the recording was played over and over in court. Zimmerman had cuts and scrapes on his face and the back of his head, but prosecutors suggested the injuries were not serious.

To secure a second-degree murder conviction, prosecutors had to convince the jury that Zimmerman acted with a "depraved" state of mind — that is, with ill will, hatred or spite.

To win a manslaughter conviction, prosecutors had to convince the jury only that Zimmerman killed without lawful justification.