U.S. President Barack Obama has called for "calm reflection" amid protests fueled by the acquittal of Florida neighborhood watch leader George Zimmerman, who was accused of killing a black teenager, Trayvon Martin.

"We are a nation of laws, and a jury has spoken," said the president in a statement posted on the White House web site  on Sunday.

Hundreds of demonstrators marched across cities in the country questioning "What would have happened if Zimmerman was a black man and Martin was a white man?"

The president in his statement called the death of Trayvon Martin as a "tragedy."

"Not just for his family, or for any one community, but for America," said Obama.

"We should ask ourselves if we're doing all we can to stem the tide of gun violence that claims too many lives across this country on a daily basis... As citizens, that's a job for all of us. That's the way to honour Trayvon Martin," added the U.S. president.

Zimmerman was cleared of second-degree murder and manslaughter charges for the death of 17-year-old Trayvon.

The jury's verdict said George shot him dead in self defense while prosecutors had argued that he had racially profiled Martin while the African-American walked through his neighborhood that led to a fight where Martin was shot by Zimmermanon on Feb. 26, 2013.

As the case sparked fierce debate in the country about racial profiling, the Department of Justice said it was investigating if a civil case could be brought against Zimmerman.

 A petition has been also launched by the National Association for the Advancement of Colored People (NAACP) urging the department of justice to open a civil rights case against the gunman and till Sunday midday, the petition has received more than 350,000 signatures, according to the BBC News.

Though Zimmerman was declared not guilty, his family members feared that he might fall victim to revenge attacks.

Zimmerman's brother Robert told BBC that he received frequent threats on social media and there was "more reason now than ever to think that people are trying to kill him".

"He's going to be looking over his shoulder the rest of his life," said Robert.