Oscar Pistorius has told a South African court that his girlfriend died in his arms, blood pouring from her body, as he tried to revive her after shooting her by mistake.

Facing questioning from prosecutor Gerrie Nel for the first time in his murder trial, Mr Pistorius admitted killing Reeva Steenkamp at his home on 14 February last year but said "I made a mistake".

The double amputee Olympic and Paralympic sprinter, once revered the world over for his triumph over adversity, faces life in prison if convicted of the murder of Ms Steenkamp, a 29-year-old law graduate and model.

His defence hinges on his contention that he though he was firing at an intruder when shot Ms Steenkamp through a toilet door in his luxury Pretoria home.

The prosecution aims to show he was a hot-headed character who loved to play with guns.

Mr Pistorius, fighting back tears, told the court how he desperately tried to revive Ms Steenkamp, who was hit by three of four pistol rounds he fired through the door.

"I checked to see if she was breathing and she wasn't," he said. "I could feel the blood was running down on me," Mr Pistorius said.

Pathologists said she was mortally wounded by one shot to the head.

He eventually managed to carry Ms Steenkamp down stairs where neighbours tried to administer first aid before paramedics arrived.

Mr Pistorius said he knew that Ms Steenkamp, with whom he said he was planning to buy a house, was already dead.

"The paramedics arrived. They asked for some space to work so I stood up," he said, his voice quavering with emotion.

"Reeva, Reeva had already died whilst I was holding her, before the ambulance arrived, so I knew there was nothing they could do for her."

With no direct witnesses, Mr Nel's main task is to pick holes in Mr Pistorius' version of events and cast doubt on the veracity of his testimony about a perceived burglar.

He opened his case by asking Mr Pistorius about his international reputation and strong Christian beliefs before hitting him with the reality of what took place.

"You are a model for sportsmen, disabled and able-bodied sportsmen, all over the world?" Mr Nel asked.

"I think I was, My Lady. I made a mistake," replied Mr Pistorius, answering to judge Thokozile Masipa, only the second black woman to ascend to the South African bench.

"You killed a person, that's what you did," came the reply.

"I made a mistake. My mistake was that I took Reeva's life," the 27-year-old said.

"You killed her. You shot and killed her. Won't you take responsibility for that?" Mr Nel said.

Mr Nel then went on to ask whether Mr Pistorius, well-known as a weapons enthusiast, knew what a 'zombie stopper' was, to which Mr Pistorius replied 'No'.

After a brief adjournment, the court then viewed video footage broadcast on Britain's Sky News of Mr Pistorius firing a handgun at a water-melon at a shooting range.

As the melon disintegrates, a male voice off-camera that sounds like Mr Pistorius says: "It's a lot softer than brains. But (bleep) it's like a zombie stopper."

Mr Pistorius admitted it was his voice, leading to Mr Nel to press him on his motives for wanting to see the water melon explode.

"You know that the same happened to Reeva's head. It exploded. I'm going to show you," he said, before projecting a forensic photograph of Ms Steenkamp's head, the side and back of her head matted with blood and brains, on the court monitors.

"Take responsibility for what you have done," he told Mr Pistorius, eliciting gasps from the packed public gallery.

Mr Pistorius hid his head in his hands in the witness stand, rocking from side to side, sobbing and saying he took responsibility "but I will not look".