SANFORD, Fla. -- After a year and a half of living as a hermit, George Zimmerman emerged from a Florida courthouse a free man, cleared of all charges in the shooting of 17-year-old Trayvon Martin.

His brother said the former neighbourhood watch volunteer was still processing the reality that he wouldn't serve prison time for the killing, which Zimmerman, 29, has maintained was an act of self-defence. A jury found him not guilty of second-degree murder late Saturday night and declined to convict him on a lesser charge of manslaughter.

However, with many critics angry over his acquittal, his freedom will likely be limited.

"He's going to be looking over his shoulder the rest of his life," Robert Zimmerman Jr. said during an interview on CNN.

Martin's killing in February 2012 unleashed furious debate across the U.S. over racial profiling, self-defence and equal justice. Protesters across the country lashed out against police in the Orlando suburb of Sanford, outraged that it took 44 days for Zimmerman to be arrested. Many, including Martin's parents, claimed Zimmerman had racially profiled the unarmed black teen. Zimmerman identifies himself as Hispanic.

Six anonymous female jurors -- all but one of them white -- 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.

They deliberated more than 15 hours over two days before announcing late Saturday night that they had reached a verdict.

Defence attorney Mark O'Mara said in August 2012 that Zimmerman and his wife, Shellie, had been living "like a hermit" and were not working because they feared for their safety.

After Saturday's verdict, police, officials and civil rights leaders urged peace and told protesters not to resort to violence. While defence attorneys said they were thrilled with the outcome, O'Mara suggested Zimmerman's safety would be an ongoing concern.

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

Those watching the case reacted strongly when the not-guilty verdict was announced. 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 verdict.

Andrew Perkins, 55, a black resident of Sanford, angrily asked outside the courthouse: "How the hell did they find him not guilty?"

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

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

Protesters had taken to the streets late Saturday and into Sunday morning in Florida, Atlanta and San Francisco, among other places. The demonstrators seemed to largely heed the advice of officials and others who urged them not to resort to violence.

Martin family attorney 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. However, he said, "for Trayvon to rest in peace, we must all be peaceful."

Martin's family maintained the teen was not the aggressor, and prosecutors suggested Martin was scared because he was being followed by a stranger. Defence attorneys, however, claimed Martin knocked Zimmerman down and was slamming the older man's head against the concrete sidewalk when Zimmerman fired his gun.

Prosecutors called Zimmerman a liar and portrayed him was 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.

Zimmerman also had some supporters outside the courthouse, including Cindy Lenzen, 50, of Casslebury, and her brother, 52-year-old Chris Bay, who stood watching others 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."

Before a special prosecutor assigned to the case ordered Zimmerman's arrest, thousands of protesters had 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. 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 neighbour'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.

Prosecutors said he demonstrated that when he muttered, "F------ punks. These a-------. They always get away" during a call to police as he watched Martin walk through his neighbourhood.

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