A six-woman jury late Saturday found neighborhood watchman George Zimmerman not guilty of murdering unarmed black teenager Trayvon Martin, in a racially-charged trial that has transfixed the United States.

"Mr. Zimmerman, I have signed the judgment that confirms the jury's verdict. Your bond will be released. Your GPS monitor will be cut off when you exit the courtroom over here. And you have no further business with the court," Judge Deborah Nelson said shortly after the decision was read.

Zimmerman smiled briefly but did not appear emotional as the verdict was read. His family was grinning broadly as they sat behind him in the courtroom.

The family of Trayvon Martin was not present.

Scores of journalists and crowds of demonstrators had gathered outside the courthouse in the dark, awaiting the verdict. Many of the demonstrators were chanting, "no justice, no peace."

The jury had deliberated for more than 16 hours since Friday in the case.

Zimmerman, 29, had been accused of pursuing Martin through a gated community in Sanford, Florida and shooting him during an altercation.

Defense lawyers insisted Zimmerman acted in self-defense after Martin, 17, wrestled him to the ground and started bashing his head into the pavement.

Zimmerman faced possible life in prison if convicted of second-degree murder. The jury was also instructed to consider an alternate charge of manslaughter, which carries up to a 30-year sentence.

The February 2012 killing ignited widespread controversy after police initially declined to press charges against Zimmerman.

Community leaders on Friday had called for calm regardless of the eventual verdict.

"If Zimmerman is convicted, there should not be inappropriate celebrations, because a young man lost his life," Reverend Jesse Jackson, a veteran civil rights activist, said.

"If he is not convicted, we should avoid violence because it will only lead to more tragedies."