SANFORD, United States / Florida (AFP) - The six-woman jury in the racially-charged trial of a US neighbourhood watchman accused of murdering unarmed black teenager Trayvon Martin resumed its deliberations on Saturday.

George Zimmerman, 29, a volunteer night watchman, is accused of pursuing Martin through the gated community of Sanford, Florida and shooting him during an altercation.

Defence lawyers insist Zimmerman acted in self-defence after Martin, 17, wrestled him to the ground and started bashing his head into the pavement.

"He's not guilty of anything but protecting his own life," lead defence attorney Mark O'Mara said in his closing statement on Friday, before jurors adjourned for around three and a half hours.

The jury resumed its deliberations at around 9am (9pm Singapore time) Saturday and was still meeting five hours later. They must reach a unanimous verdict to convict.

Zimmerman, whose father is white and whose mother is Peruvian, has denied any racial motive, saying he was suspicious because the neighbourhood had seen a recent string of break-ins.

But prosecutors say Zimmerman, who pursued Martin against the advice of a police dispatcher, instigated the confrontation.

"He profiled him as a criminal. He assumed... that Trayvon Martin was up to no good. And that's what led to his death," prosecutor Bernie de la Rionda told jurors in his closing arguments on Thursday.

Judge Nelson reminded the jury on Friday that citizens have the right to kill in self-defence, and that the burden was on the prosecution to prove Zimmerman committed murder.

Zimmerman faces possible life in prison if convicted of second-degree murder. The jury could alternatively find him guilty of manslaughter, which carries up to a 30-year sentence.

The February 2012 killing ignited widespread controversy after police initially declined to press charges against Zimmerman.

The trial has received close to non-stop coverage by all three major US cable broadcasters.

Supporters of Martin, who had no criminal record and was walking back to a family friend's house after buying an Arizona ice tea and Skittles, insist he was targeted because he was young, black and wearing a hooded sweatshirt.

The "hoodie" has since become a symbol of opposition to racial profiling, with black US congressman Bobby Rush wearing one on the House floor during a March 2012 speech denouncing the initial handling of the incident.

President Barack Obama also weighed in on the controversy, calling the shooting "a tragedy" and suggesting that, if he'd had a son instead of two daughters, "he'd look like Trayvon."

In demonstrations outside the courtroom on Saturday, around a dozen pastors and black activists carrying Skittles and Arizona ice teas demanded "justice" for Martin while a handful of white demonstrators insisted Zimmerman had acted in self-defence.

"This was a racial murder and we want justice," said Tracy Carpio, a 22-year-old University of Florida student, as Martin supporters chanted: "It's neighbourhood watch, not neighbourhood killer."

Flora Reece, a black woman in her 40s who recently moved to Florida from Texas, said Zimmerman killed Martin "because he was a black young man in a hoodie. He profiled him."

"Florida is the most racist state in the US and black males are an endangered species here," she said.

"I can tell you, as a black woman, I possess a bachelor's degree and two master's degrees, but I am profiled daily in this state." On the other side, Patrick Woodburn, a 52-year-old Orlando resident, said he was convinced Zimmerman would go free because he had acted in self-defence.

"If Trayvon Martin had been in the role of Zimmerman I'd be here supporting him, because I support the rule of law," said Woodburn, who is white.

"These black activists say it is racism, but they forget that racism is part of the history of our country and exists in both directions." Community leaders on Friday called for calm regardless of the eventual verdict.

"If Zimmerman is convicted, there should not be inappropriate celebrations, because a young man lost his life," Reverend Jesse Jackson, a veteran civil rights activist, said.

"If he is not convicted, we should avoid violence because it will only lead to more tragedies."