The trial aroused strong passions among those who believed that Zimmerman -- a volunteer neighbourhood watchman whose father is white and whose mother is Peruvian -- racially profiled and stalked Martin, and those convinced he acted in self-defence.
Spontaneous protests broke out overnight in US cities including San Francisco, Philadelphia, Chicago, Washington and Atlanta, with larger organised gatherings planned later today.
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 specter of the deadly April 1992 riots in Los Angeles, which broke out after a similarly 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 outside the courthouse was loud at times, but peaceful.
Defence attorney Don West was even more blunt, calling the prosecution of Zimmerman "disgraceful."
Defence lawyers insisted Zimmerman feared for his life after Martin attacked him, pinned him to the ground and started slamming his head against the pavement.
Zimmerman is the only living witness to how the fight began.
"Even though I am broken-hearted my faith is unshattered I WILL ALWAYS LOVE MY BABY TRAY," Martin's father Tracy wrote on Twitter.