MIAMI, Florida (AFP) - The mother of Trayvon Martin said on Monday the volunteer neighbourhood watchman who shot her unarmed son got away with murder thanks to a controversial self-defence law.

George Zimmerman was acquitted of killing the 17-year-old Martin following a trial that stirred up controversy on "stand your ground" laws, which assert citizens can use lethal force if they fear their lives are in danger, even if they might have been able to safely retreat.

"The thing about this law is I just think it assisted the person who killed my son to get away with murder," Ms Sybrina Fulton told a National Bar Association conference in Miami Beach.

"I think we have to change these laws so people don't get away with murder," she added.

Martin was walking through a gated Florida community, carrying iced tea and sweets, when Zimmerman fatally shot him after a confrontation.

Zimmerman's trial lawyers did not explicitly invoke the "stand your ground" law during the trial, arguing instead that he acted in self-defence in the traditional sense.

But the case stirred up controversy around such laws, present in some 30 states.

US Attorney General Eric Holder has urged a rethink of such laws. However, Florida Governor Rick Scott has said he will not seek a rollback.

Civil rights leaders have vowed to use Florida as a testing ground to challenge and change "stand your ground" laws, which they feel disproportionately impact African Americans and people of colour.