In a blistering start to his cross-examination, prosecutor Gerrie Nel wasted no time in putting the Olympic athlete on the back foot, demanding he accept responsibility for his actions on the morning of Valentine's Day 2013.

"You shot and killed her ... Say it: 'I shot and killed Reeva Steenkamp,' " Mr Nel demanded in Pretoria's High Court.

"Yes I did, my lady," Pistorius said in a small voice.

But the barrage did not stop there.

Mr Nel asked the screens in the court to show a video clip, previously aired on Sky News in South Africa, in which the double amputee fires bullets at a watermelon at a shooting range.

When one of his shots destroys the fruit, causing it to explode, Pistorius can be heard saying: "It's a lot softer than brains" and "it's like a zombie stopper".

Then without warning, an image of Ms Steenkamp's bloody skull flashed up on the screen as Mr Nel said: "You know that the same happened to Reeva's head? It exploded."

Gasps echoed around the court as the images, which were also broadcast live on TV, brought the Olympian undone.

He shouted tearfully: "I won't look at that picture. I remember! I was there!"

He said he was "tormented by that" image.

"My fingers touched her head," he said, crying.

Burying his head in his hands, he dissolved into sobs, as his barrister Barry Roux objected, saying the prosecutor had gone "too far".

Judge Thokozile Masipa agreed, asked the image be taken down, and the court swiftly adjourned, with Pistorius was once more inconsolable.

As Pistorius sat hunched over in the witness box, his sister Aimee rushed to his side, shooting an angry and upset look in the direction of the prosecutor as she did so.

Ms Steenkamp's mother, June, told reporters in the break she understood why the picture had to be shown, and had been warned about it in advance.

She had simply put her head down, seemingly crying, as the image was held on the screens.

When the court resumed, Mr Nel returned to asking Pistorius about the watermelon incident, suggesting he "shot that firearm to see what the effect would be" of shooting someone in the head.

Pistorius denied the inference, saying the distasteful remark was about zombies.

When Mr Nel had got to his feet shortly before 11am to begin his eagerly anticipated cross-examination, he began by asking Pistorius to agree he was one of the most "recognised faces in the world" and "a model for disabled sportsmen and able-bodied sportsmen, all over the world".

"I think I was, my lady, until that terrible mistake," he replied.

Mr Nel seized on the word "mistake".

"You killed a person, that's what you did, isn't it? You killed Reeva Steenkamp, that's what you did?"

Pistorius repeated the phrase, and Mr Nel asked him to explain what that "mistake" had been.

"My mistake was that I took Reeva's life," he said.

After lunch, Mr Nel began a meticulous dissection of the Blade Runner's version of events, honing in on slight differences in what Pistorius said at his bail application, in his "plea explanation" at the start of the trial, and in his evidence-in-chief.

He suggested he had tailored his version to suit the evidence uncovered by the state.

When questioning him about the fans Pistorius said he had brought in from the balcony, the athlete said it was not a big deal and he could not remember where the fans had been plugged in on the night he shot and killed Ms Steenkamp.

"It's not insignificant, Mr Pistorius," Mr Nel said. "It will show that you are lying."

Pistorius also said he had "never intended to kill anyone".

"I had my finger on the trigger, it was an accident that happened … I didn't intend to shoot anyone. I fired my firearm before I could think, before I even had a moment to comprehend what was happening.

Earlier, Pistorius battled against rising emotions as he resumed his testimony from Tuesday, describing how he carried the bloody body of his girlfriend downstairs after shooting her through a toilet door.

"I had my fingers in her mouth I was trying to help her breathe," Pistorius said.

"I had my hand on her hip. I was trying to stop the bleeding."

"She already died while I was holding her," Pistorius said.

"Every time I looked up there were more people in the house, there were more policemen. I asked a policeman if I may wash my hands because the smell of the blood was making me throw up."

Shortly before the lunch adjournment, Mr Nel began questioning Pistorius about discrepancies between the version he gave to his bail hearing in February 2013, shortly after the incident, his "plea explanation" from the beginning of the trial and his evidence.

The case continues.