The trial raised strong passions among those who believed that Zimmerman -- whose father is white and whose mother is Peruvian -- thought Martin was a criminal just because he was black, and those convinced that the volunteer watchman acted in self-defence.
Spontaneous protests broke out overnight in US cities including San Francisco, Philadelphia, Chicago, Washington and Atlanta.
In Oakland, California, protesters smashed windows and spray painted cars, but most overnight demonstrations were peaceful -- and closely watched by police.
Florida police initially declined to press charges against Zimmerman, sparking mass protests in several US cities. He was eventually arrested in April 2012 and charged with second-degree murder and manslaughter.
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The spectre of the deadly April 1992 riots in Los Angeles, which broke out after a similar racially-charged case, still lingers among law enforcement officials.
Fearing 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.
Defense attorney 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.
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.