His voice trembling with emotion, Oscar Pistorius took the witness stand in his own defense on Monday, saying the Valentine's Day shooting of his girlfriend last year had left him sleepless, terrified and plagued by nightmares.

The disabled South African track star, on trial for murder, apologized to the mother of model Reeva Steenkamp, saying he had fired four times through a toilet door at his luxury Pretoria home in the belief he was defending her from an intruder.

Steenkamp, a law graduate and model, was hit by at least three rounds, one of which - to the head - killed her almost instantly, the court has heard.

"I was simply trying to protect Reeva," Pistorius told the Pretoria High Court at the start of his testimony. Reeva's mother June Steenkamp, sitting stony-faced in the packed public gallery, bowed her head.

The 27-year-old Olympic and Paralympic star, who faces life in prison if convicted of murder, testified he had been on anti-depressants and sleeping pills because of his disturbed state of mind since the shooting.

"I'm scared to sleep. I have terrible nightmares about things that happened that night," he said.

"I can smell blood. I wake up to being terrified."

He recounted one occasion when he woke up so scared in the middle of the night that he crawled into a cupboard before calling his sister, who came round to sit with him.

"I wake up in a complete state of terror to the point that I would rather not sleep," he said, adding that his faith had helped pull him through.

"There have been times when I've just been struggling a lot," he said, fighting to maintain his composure.

"My god is my god of refuge."

Earlier, during graphic forensic testimony from a defense pathologist, Pistorius retched into a bucket in the dock.

OBSESSION WITH GUNS

The distraught, bespectacled figure was in stark contrast to the gun-obsessed, fast-living hothead that prosecutors had described in the first 16 days of the trial.

As well as murder, Pistorius is accused of firing a pistol through the sun roof of a friend's car while on a public road, and discharging a handgun under the table of a packed Johannesburg restaurant.

He has pleaded not guilty to all charges.

Now in its 17th day, the trial has gripped South Africa and millions of fans around the world who have seen in Pistorius a symbol of triumph over physical adversity.

The sprinter's lower legs were amputated as a baby but he went on to achieve global fame as the "Blade Runner", after the slender carbon fiber prosthetic limbs he wears on the track.

After winning gold medals at the Beijing and London Paralympics, he stunned the world by reaching the semi-finals of the 400 meters in the London Olympics against able-bodied athletes.

In testimony, Pistorius described his difficult early years as a disabled child before his sporting prowess earned him respect and recognition at school.

He also spoke of the pressures of fame, the toll travelling to athletics meetings took on his family and personal life and of his fears of life in crime-ridden South Africa.

"When we were growing up, we were exposed to crime - house break-ins, family members being assaulted and hijacked," he said. "Many members of my family have been victims of house break-ins and violent crime."

"EXHAUSTED"

His defense hinges on his assertion, made under oath at his bail hearing, that he mistook Steenkamp for an intruder hiding in the toilet and opened fire.

However, witnesses living close to Pistorius' home in a gated Pretoria community have testified to hearing the terrified screams of a woman before and during a volley of shots in the early hours of February 14, 2014.

South Africa's firearms and self-defense laws make clear a person may only shoot if there is a direct threat to somebody's life - regulations Pistorius acknowledged in a written gun-license test presented by the prosecution.

Ashen-faced, Pistorius said he had not slept the night before his testimony, leading Judge Thokozile Masipa to adjourn the session early after little more than an hour of cross-examination.

"He does look exhausted," Masipa said. "He does sound exhausted." As he stepped from the witness box, Pistorius slumped into the outstretched arms of his sister Aimee.

The trial resumes at 0730 GMT on Tuesday.