Graphic photographs of Trayvon Martin's body lying face-up on the grass of a Florida housing estate were shown to a jury Tuesday afternoon as testimony continued in neighbourhood watch leader George Zimmerman's second-degree murder trial.

The pictures, including close-ups of Martin's face, his prostrate body and the single gunshot wound to the chest that killed him, came during the prosecution's questioning of Diana Smith, a crime scene technician for the Sanford police department who took them on the night of 26 February last year.

Martin's parents, Sybrina Fulton and Tracy Martin, left the Sanford courtroom looking distressed soon after the photographs were displayed on a large projector screen.

The jury was also shown for the first time the 9mm semi-automatic Kel-Tec pistol that Zimmerman, 29, used to kill the 17-year-old unarmed black teenager in an altercation at the Retreat at Twin Lakes gated community. He denies murder on the grounds of self-defence.

Smith said she arrived on the scene at about 7.55pm to find Martin's body lying under a yellow blanket and the area taped off by police officers.

She described identifying and photographing items near the body, including a bullet casing, Martin's keys, cellphone, headphones and a bag of Skittles he had just bought at a 7-Eleven convenience store less than a mile away.

Later that evening, she said, she returned to police headquarters and took photographs of Zimmerman, who was being questioned but was not under arrest. Those pictures, also displayed in the courtroom on Tuesday, show Zimmerman in a red jacket, light-coloured shirt and jeans.

Close-ups showed a cut on his nose and bloody lacerations on the back and top of his head. Defence attorney Don West said in opening arguments on Monday that Martin "sucker-punched" Zimmerman, straddled him and smashed his head on a concrete pavement, forcing his client to fire to save his own life.

Earlier, Sanford police sergeant Anthony Raimondo described his futile efforts to save Martin, using a plastic grocery bag to try to seal the gunshot wound in the teenager's chest from which he heard bubbling caused by escaping air.

Raimondo, one of the first officers to arrive, said he found Martin lying face down with his hands under his body, and turned him over with the help of another officer so he could try mouth-to-mouth resuscitation.

"I breathed for Mr Martin, or tried to," he said, adding that he ignored the department's usual policy of using a protective mask because of the "rare and extraordinary" circumstances of the situation.

The bubbling sounds, he said, "meant that air was getting into or air was escaping from the chest in a manner it's not supposed to." He added he could not find an exit wound for the bullet, so he was unable to make an effective

seal with the plastic bags given to him by a resident.

"I put an emergency blanket over Mr Martin's body, it was respect for the deceased, to mitigate trauma for family members if they arrived on scene and to preserve any physical evidence on the body," he said.

Prosecutors have portrayed the defendant as a neighbourhood busybody and an overzealous vigilante who profiled, pursued and shot Martin as he walked through the development to the house of his father's friend in a hooded top.

"These assholes, they always get away," he told a police dispatcher after calling to report "a suspicious male" in the neighbourhood.

Wendy Dorival, the volunteer programme co-ordinator for the city's police department, however, said she found Zimmerman to be always "polite, courteous and respectful".

"He seemed like he really wanted to make changes in his community, to make it better. I wanted to recruit him to be a citizen on patrol volunteer, but he said no," she said.

Dorival said Zimmerman told her that his homeowners association had asked him to lead efforts to establish a neighbourhood watch. She said she gave a presentation to about 25 residents, stressing that citizens are told not to follow or confront anyone they considered suspicious.

Don O'Brien, president of the homeowners association of the Retreat at Twin Lakes, disputed that Zimmerman was asked or appointed to form a neighbourhood watch. "I found out there was a course he went through with the Sanford police department to start a watch. We had nothing to do with it," he said.

"He went to the police, he got this whole thing started, it was his programme so he was running it."

The first resident of development to take the stand, Selene Bahadoor, said she heard noises outside and saw two individuals upright "with flailing arms" but it was too dark to identify anybody.

Zimmerman faces at least 25 years in jail if convicted after a trial expected to last between two and four weeks.