The US president has called for calm after a Florida man accused of shooting an unarmed teenager was found not guilty of murder.

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Zimmerman verdict: America reacts. Photo: Getty Images.

US President Barack Obama has urged for “calm reflection” after a jury found George Zimmerman, a neighbourhood watch volunteer, not guilty of second-degree murder for shooting death of black teenager Trayvon Martin.

The acquittal sparked protest marches overnight in San Francisco, Chicago, Washington, Philadelphia and Atlanta. In Oakland, California, protesters smashed windows and spray painted cars.

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“The death of Trayvon Martin was a tragedy. Not just for his family, or for any one community, but for America,” said Obama. "I know this case has elicited strong passions. And in the wake of the verdict, I know those passions may be running even higher. But we are a nation of laws, and a jury has spoken.”

Instead, he urged Americans to join the fight against gun violence. “We should ask ourselves, as individuals and as a society, how we can prevent future tragedies like this.”

Zimmerman fatally shot the 17 year old on Feb 26, 2012. Prosecution argued that Zimmerman had targeted the teen, who was later found to be carrying only a packet of lollies and a mobile phone, while Martin said that he had shot in self-defence.

The case has sparked a passionate debate about racial profiling in the US.