A Florida jury found neighbourhood watchman George Zimmerman not guilty of murdering unarmed black teenager Trayvon Martin in a racially charged trial that transfixed the nation.
The trial raised strong passions among those who believed that Zimmerman - whose father is white and whose mother is Peruvian - had racially profiled Martin, and those convinced that the volunteer watchman acted in self-defence.
Zimmerman, 29, had been accused of pursuing Martin, 17, through a gated community in Sanford, Florida, and shooting him during an altercation on a rainy night on February 26, 2012.
The killing led to mass marches in several US cities after police initially declined to press charges against Zimmerman.
Fearing more protests and outbreaks of violence after the verdict, activists and community leaders appealed for calm. Police were out in force in Sanford, and the crowd of several hundred gathered outside the courthouse was loud at times but peaceful.
Spontaneous protest marches however were staged overnight in US cities including San Francisco, Philadelphia, Chicago, Washington and Atlanta, US media reported.
"Obviously, we are ecstatic with the results. George Zimmerman was never guilty of anything except protecting himself in self-defence," said his lead lawyer Mark O'Mara after the verdict.
Defence lawyer Don West was even more blunt. "I think the prosecution of George Zimmerman was disgraceful," he said.
Defence lawyers insisted that Zimmerman acted in self-defence after Martin wrestled him to the ground and slammed his head on the pavement.
Zimmerman smiled briefly but did not appear emotional as the verdict was read. His family grinned broadly as they sat behind him in the courtroom.
According to Florida's controversial "Stand Your Ground" law, people who fear for their lives can use deadly force to defend themselves without having to flee a confrontation.
"Even though I am broken-hearted my faith is unshattered I WILL ALWAYS LOVE MY BABY TRAY," Martin's father Tracy wrote on Twitter.
Both he and Martin's mother Sybrina Fulton gave thanks for the outpouring of support they had received over the past year.
Zimmerman's older brother Robert also took to Twitter: "Message from Dad: 'Our whole family is relieved.' Today... I'm proud to be an American. God Bless America! Thank you for your prayers!" he tweeted.
Community leaders called for calm after the verdict.
"Avoid violence, it will lead to more tragedies. Find a way for self construction not deconstruction in this time of despair," wrote civil rights leader Jesse Jackson on Twitter.
Activist Al Sharpton posted a statement on Facebook describing Zimmerman's acquittal as "a slap in the face to the American people," and said that he was convening "an emergency call with preachers (to) discuss next steps."
The NAACP, the largest US civil rights group, urged supporters to sign a letter asking Attorney General Eric Holder to file civil charges against Zimmerman.
"The most fundamental of civil rights - the right to life - was violated the night George Zimmerman stalked and then took the life of Trayvon Martin," read the NAACP letter. "We ask that the Department of Justice file civil rights charges against Mr. Zimmerman for this egregious violation."
Prosecutors argued that Zimmerman, who pursued Martin against the advice of a police dispatcher, instigated the confrontation.
The all-female jury had to reach a unanimous verdict to convict or acquit, and deliberated for more than 16 hours on the case after closing arguments by the two sides on Friday.
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 a sentence of up to 30 years.
"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.
Florida Prosecutor Angela Corey argued that the tragic case was a test of Florida's gun laws as well as social boundaries.
"This case has never been about race, nor has it ever been about the right to bear arms," Corey said. "But Trayvon Martin was profiled. There is no doubt that he was profiled to be a criminal. And if race was one of the aspects in George Zimmerman's mind, then we believe that we put out the proof necessary to show that Zimmerman did profile Trayvon Martin."