US Attorney General Eric Holder strongly criticised "stand-your-ground" laws that allow a person who believes he is in danger to use deadly force in self-defence, even in public places.
Speaking on Tuesday, Holder said he was concerned about the case of Trayvon Martin which has captivated and polarised the US public on issues of race, gun and self-defense laws.
"Separate and apart from the case that has drawn the nation's attention, it's time to question laws that senselessly expand the concept of self-defence and sow dangerous conflict in our neighborhoods," Holder said.
On Saturday, George Zimmerman, 29, was acquitted of second degree murder and manslaughter charges after a jury of six anonymous women found him not guilty.
Holder said the Justice Department has an open investigation into what happened.
In the meantime, after a spate of vandalism and violence, Los Angeles police vowed to crack down with quick action and arrests if further disturbances arose from street protests over the acquittal.
In Washington, civil rights activist Reverend Al Sharpton called for protests at the Justice Department offices and federal courthouses across the country.
Zimmerman, a neighbourhood watch volunteer, said he shot the unarmed Martin, a black youth, in self-defence.
He is now said to be in hiding.
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"People all over the country will gather to show that we are not having a two- or three-day anger fit," said Al Sharpton, a preacher, television host and civil rights advocate joined by about 15 other clergy.
Sharpton said demonstrations were planned for Saturday in 100 cities to push for new charges against Zimmerman and the repeal of Florida's "stand your ground" self-defense law.
"We don't need consolation. We need legislation, and we need some federal prosecution," Sharpton said.
Any charges against Zimmerman would likely fall under a 2009 law against hate crimes, but lawyers with civil rights expertise said a new prosecution was unlikely because of the lack of evidence that racism drove Zimmerman to shoot.
A juror in Zimmerman's trial told CNN she did not think Zimmerman racially profiled Martin.
"All of us thought race did not play a role," said the juror, granted anonymity by the television news network.
'They always get away'
Protesters at demonstrations following the verdict have accused Zimmerman, who is white and Hispanic, of racially profiling Martin.
Sharpton said he was aware of the legal questions, but he said Zimmerman had a pattern of profiling black men as criminal suspects.
He also quoted Zimmerman's lament of "they always get away" to a police dispatcher upon seeing Martin.
"Who is they? And get away with what, since all he (Martin) was doing was going home?" Sharpton said.
Zimmerman's defense lawyers have said their client was viciously attacked by Martin.
The black preachers said they considered Martin's death a spark for a renewal of the US civil rights movement by a generation that grew up after the desegregation struggles of the 1950s and 1960s.
"There is going to be an intellectual riot," said Jamal Bryant, pastor of the Empowerment Temple church in Baltimore, Maryland.
"One hundred cities are going to feel the wrath of a guided, intellectual, meaningful, spiritual uprising."