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 neighborhood watch volunteer was still processing the reality he wouldn't serve prison time for the killing, which Zimmerman, 29, has maintained was an act of self-defense. 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.

Demonstrators upset with the verdict protested mostly peacefully in Florida and Atlanta overnight, but some broke windows and vandalized a police squad car in Oakland during protests in four California cities, authorities said.

"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 US over racial profiling, self-defense 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 on Saturday night that they had reached a verdict.

Defense attorney Mark O'Mara said in August that Zimmerman had been living "like a hermit", and he and his wife, Shellie, 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 defense attorneys said they were thrilled with the outcome, O'Mara suggested Zimmerman's safety will 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 reacted strongly when the verdict was announced. Martin's mother and father were not in the courtroom when it was read, but 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?"

The Associated Press