Oscar Pistorius became distraught as he was shown a graphic photo of his dead girlfriend Reeva Steenkamp by the prosecution at hismurder trial.
"It's time that you look at it," chief prosecutor Gerrie Nel said, presenting the photo at the start of the first day of cross-examination.
The South African athlete, who denies murdering Ms Steenkamp, insisted he had made a "mistake".
He says he accidentally killed her after mistaking her for an intruder.
He told the court on Wednesday: "I made a mistake. My mistake was that I took Reeva's life."
Prosecutors say Mr Pistorius intentionally killed Ms Steenkamp on 14 February last year by firing a gun through a bathroom door at her after a row at his Pretoria home.
The 27-year-old Olympic and Paralympic sprinter, who is a double amputee, faces life imprisonment if convicted of murdering the 29-year-old model, reality TV celebrity and law graduate.
Following legal arguments on Wednesday morning, Mr Nel showed video footage of the athlete at a gun range shooting a watermelon.
Mr Nel then said the watermelon had exploded in the same way as Ms Steenkamp's head - and showed the graphic photo. He asked Mr Pistorius to look at it and take responsibility for his actions.
An emotional Mr Pistorius turned away from the image on a monitor screen and said: "I've taken responsibility… but I will not look at a picture where I am tormented by what I saw and felt that night... I remember. I don't have to look at a picture, I was there.''
After an adjournment to allow Mr Pistorius to compose himself, Mr Nel sought to highlight what he said were inconsistencies in the athlete's statements, questioning his claims that police had tampered with items in his home after the shooting.
"I am not pleading not guilty because the scene was contaminated," Mr Pistorius countered. "I am pleading not guilty because what I'm accused of didn't happen."
The BBC's Andrew Harding, who was in court in Pretoria, says in the opening moments of cross examination, Mr Nel showed the highly combative style Mr Pistorius can now expect from him in the coming days.
Earlier, while being questioned by his lawyer Barry Roux, Mr Pistorius told the court he had 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."
He said he had managed to carry Ms Steenkamp downstairs, where neighbours had tried to administer first aid before paramedics arrived.
"The paramedics arrived. They asked for some space to work so I stood up," Mr Pistorius said. "Reeva had already died whilst I was holding her, before the ambulance arrived, so I knew there was nothing they could do for her."
Before Mr Roux ended his questioning, he asked Mr Pistorius if he had intentionally killed Ms Steenkamp.
"I did not intend to kill Reeva or anybody else for that matter," he replied.
If Mr Pistorius is acquitted of murder, South African law stipulates that the court must consider the separate, lesser charge of culpable homicide, or manslaughter, for which he could receive between six and 15 years in prison.
Mr Pistorius also faces charges of illegally firing a gun in public and of illegally possessing ammunition, both of which he denies.
There are no juries at trials in South Africa, and his fate will ultimately be decided by the judge, assisted by two assessors.