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Taking the witness stand, the double amputee - accused of murdering 29-year-old law graduate and model Reeva Steenkamp - recounted hearing a window sliding open in his bathroom in the middle of the night on Valentine's Day 2013.
Conditioned by years of living in crime-ridden South Africa, Pistorius said the noises convinced him someone was breaking into his luxury Pretoria home and that he needed to protect himself and his lover.
"That's the moment that everything changed," he said, his voice tense with emotion. "I thought that there was a burglar that was gaining entry to my home."
"The first thing that ran through my mind was that I needed to arm myself, that I needed to protect Reeva and that I needed to get my gun," said the 27-year-old, who faces life in prison if convicted of murder.
Pistorius said he felt in the dark with his fingers, grabbed a 9mm pistol lying under the bed, then moved on his stumps down the passageway leading from the bedroom towards the bathroom and the would-be intruder or intruders.
"I shouted for Reeva to get on the floor and for her to phone the police. I screamed at the people to get out," he said.
According to his account, as he peered round the door of the bathroom - his outstretched right hand holding the pistol, his left hand steadying himself against the wall - he noticed the bathroom window was open, confirming his worst fears.
"I wasn't sure where to point my firearm. I had it pointed at the toilet but my eyes were going between the window and the toilet. I stood there for some time. I'm not sure how long.
"I just stayed where I was and kept on screaming. Then I heard a noise from inside the toilet that I perceived to be somebody coming out of the toilet. Before I knew it, I had fired four shots at the door."
With his ears ringing, he continued to scream for Steenkamp to call the police, he said, until it slowly dawned on him that his girlfriend might have been the one behind the door.
"I was screaming and shouting the whole time. I don't think I have ever screamed or cried like that. I was crying out to the Lord to help me. I was crying out to Reeva," he said, choking back the tears.
He then recounted how he bashed out a panel of the wooden door with a cricket bat to reveal Steenkamp - the woman with whom he said had been planning on buying a house - slumped on the floor in a pool of blood.
"I sat over Reeva and I cried," he said, before breaking down into uncontrollable sobs, causing Judge Thokozile Masipa to adjourn the hearing for the day.
Pistorius' testimony was broadly in line with a sworn affidavit he gave at his bail hearing just over a year ago. At the time, magistrate Desmond Nair pointed to what he said were a number of "improbabilities" in the version of events.
"I have difficulty in appreciating why the accused would not seek to ascertain who exactly was in the toilet," Nair said at the time. "I also have difficulty in appreciating why the deceased would not have screamed back from the toilet."
Several witnesses have testified to hearing a man's shouts coming from the house although they have also spoken of the terrified screams of a woman leading up to and during a volley of shots. Prosecutors allege that Pistorius and Steenkamp had a violent argument before he pulled a gun on her.
The trial, now in its 18th day, has gripped South Africa and millions of sports and athletics fans around the world who saw Pistorius as a symbol of triumph over physical adversity.
His disabled lower legs were amputated as a baby but he went on to achieve global fame as the "fastest man on no legs", winning numerous Paralympic gold medals and reaching the semi-finals of the 400 metres in the 2012 London Olympics against able-bodied athletes.
Earlier, defence advocate Barry Roux led Pistorius through a series of text messages between himself and Steenkamp that painted the picture of a young couple besotted with each other, nearly every message ending in 'xx' or a term of endearment.
The image was a far cry from the gun-obsessed, fast-living hothead whom prosecutors sought to portray in the first three weeks of the trial.
"I wish I could tuck you in and get you feeling better," Pistorius said in one message after Steenkamp complained of feeling under the weather. "Thank you for being the most beautiful person to me," she said in another message to him.
However, prosecutors have also used the same cache of messages retrieved from Pistorius' phone to reveal outbursts of temper and jealousy.
During his testimony, he also revealed Steenkamp's last words, spoken to him after he woke hot and uncomfortable in the early hours of Feb. 14 before going to move some fans and close the external sliding doors of his bedroom.
"'Can't you sleep, my baba?'" he quoted her as saying from the bed beside him. "I said 'No, I can't'."
As he outlined his version of events, Steenkamp's mother June, who had sat steely faced throughout Pistorius' two days on the stand, leant forward slowly and buried her head in her hands.
Edited excerpts from Pistorius' testimony
Pistorius told the court he heard his bathroom window sliding open in the middle of the night. Convinced that one or more intruders were climbing in, he picked up a 9mm pistol from under his bed and moved towards the bathroom on his stumps.
"I got to the entrance of the bathroom, at the end of the passage, where I stopped screaming. At this point I was certain that an intruder or intruders were there in my bathroom.
"I had my pistol in my right hand and I peered in.
"There was no light in the bathroom. As I slowly peered into the bathroom, I could see that the window was open indeed.
"At this point I started screaming again for Reeva to phone the police ... I wasn't sure where to point my firearm. I had it pointed at the toilet but my eyes were going between the window and the toilet."
"I wasn't sure if somebody was going to come out of the toilet and attack me. I wasn't sure if someone was going to come up the ladder and point a firearm and start shooting. So I just stayed where I was and I kept on screaming."
"Then I heard a noise from inside the toilet, what I perceived to be somebody coming out of the toilet. Before I knew it, I fired four shots at the door."
"My ears were ringing, I couldn't hear anything, so I kept on shouting for Reeva to phone the police. I was so scared to retreat because I wasn't sure if there was somebody on the ladder. I wasn't sure there was somebody in the toilet.
"I don't know how long I stood there. I shouted for Reeva.
"At some point I decided to walk back to the room because I couldn't hear anything. My ears were ringing.
"I kept on shouting for Reeva, I didn't hear anything. At this point it didn't occur to me that it could have been Reeva in the bathroom. I still thought they could be intruders.
"I retreated back to the point where I got to the corner of the bed. I tried to lift myself up while talking to Reeva. No one responded to me. At that point lifted myself up into the bed and I thought Reeva was there and I couldn't feel anything.
"At that point the first thing I thought was maybe she got down onto the floor like I told her to, maybe she was just scared ... I can't remember what I said but I was trying to talk out to her.
"It was upon that time, my Lady, that it first dawned upon me that it could be Reeva that was in the bathroom or in the toilet. I jumped out of the other side of the bed and I ran my hands along the curtains to see that she wasn't hiding.
"I didn't want to believe it was Reeva in the toilet, I was so scared that someone was coming in to attack us. I made my way inside the bathroom ... I tried to grab the handle, rip open the door. I pushed the door to open and it was locked.
"I ran back to the room, I opened the curtains, opened the doors and shouted from the balcony for help. I screamed 'help, help, help'. I screamed for somebody to help me.
"I put my prosthetic legs on. I ran as far as I could back to the bathroom, I ran into the door. It didn't move at all. I tried to kick the door but nothing happened."
"I was just panicked. At this point I didn't know what to make or what to do. So I run back to the bedroom where the cricket bat was between the cabinet and the door. I was screaming and shouting the whole time. I don't think I have ever screamed or cried like that. I was crying out to the Lord to help me, I was crying out to Reeva, I was screaming.
"I run straight back to the bathroom door, I must have placed my firearm on the carpet in the bathroom. The light was on at that stage, I don't remember switching it on but it was on when I kicked the door. I run straight up to the door and I started hitting it hard. I think I hit it three times.
"The first time I remember hitting the frame of the door, there was shock in my hands, I swung again and hit a small piece open. At that point all I wanted to do was to just look inside and to see if it was Reeva. I then hit the door, I think I hit the door three times and there was a big plank, I grabbed it with my hands and I threw it out into the bathroom. I tried to open the door from the inside but there was no key on the door.
"I leaned over the middle partition of the door and I saw the key was on the floor at that point and all I wanted to do was climb into the toilet over the middle part of the door. I took the keys and unlocked door. I sat over Reeva and I cried, I don't know how long ... I don't know how long I was there for."