A sobbing Oscar Pistorius described the moment he shot and killed his girlfriend Reeva Steenkamp claiming he fired four shots "before I knew it", he told his murder trial on Tuesday.
The sprinter wept so uncontrollably as he recounted the events of the fateful night that it forced the adjournment of his trial until Wednesday.
Trembling and overwrought, the champion athlete described rushing and stumbling toward his bathroom with a gun in the dead of night on Valentine's Day 2013 after hearing a suspected intruder.
"I was overcome with fear" he said, his voice cracking with anguish. "Before I knew it, I'd fired four shots at the door".
Pistorius faces a life sentence if found guilty of deliberately murdering Steenkamp, a charge he denies.
"My ears were ringing, I didn't hear anything," Pistorius continued. He shouted for Steenkamp to phone the police, and rushed back into the room.
"I kept on shouting for Reeva."
When she did not respond Pistorius said he realised he may have made a terrible mistake and rushed back to the bathroom to break down the door.
There, in a pool of blood, he found her fatally wounded, with gunshots to the hip, arm, hand and head.
"I sat over Reeva," the athlete testified crying loudly and putting his head in his hands and before letting out the barely intelligible cry, "she wasn't breathing".
After a brief adjournment from the harrowing testimony, his lawyer Barry Roux came back to say his client was in no state to continue.
The case was adjourned until Wednesday.
- The moment 'everything changed' -
Earlier, Pistorius had explained how the couple had gone to bed around 10pm after a quiet evening, but woke up to bring in two fans he had left outside to cool his bedroom.
At that point he knew Steenkamp was awake and they exchanged a few words.
"That's the moment that everything changed," he said, after he heard the sound coming from the bathroom.
"The first thing that ran through my mind was that I needed to arm myself," he said under defence lawyer Barry Roux's gentle coaxing.
"I wasn't sure where to point the firearm. I had it pointed at the toilet but my eyes were going to the window and the toilet," he said.
"I wasn't sure if someone was going to come out of the toilet attacking me," he said, his voice quavering.
His sister Aimee was crying softly with her head bowed, while Steenkamp's mother June sat impassively in the public gallery.
- A future together -
Pistorius's initial evidence has focused on countering the prosecution's portrayal of him as reckless and obsessed with fast cars and guns.
He had begun his second day of testimony with a description of how he met the vivacious law graduate and how they quickly grew closer.
He said they had begun to plan a future together.
"The first six days we knew each other we called each other every day," he said.
"I was very keen on Reeva," he said. "We started really seeing a future with each other."
The star Paralympian was in tears as he read reams of transcribed cell phone messages he and Steenkamp sent each other during their four month relationship.
In one typical message the aspiring actress affectionately said "I love you, boo".
Pistorius also sought to explain text messages in which Steenkamp said she was frightened by his behaviour.
"I'm scared of you sometimes and how you snap at me and how you will act towards me," Steenkamp said.
Pistorius said: "I just think it was a bad hour in our relationship," describing a fight at an engagement party.
He also issued a tearful apology to Steenkamp's family.
"I can promise that when she went to bed that night she felt loved."
He is likely to remain on the stand most of the week as his extensive testimony is expected to be followed by gruelling and lengthy cross-examination by the prosecution.
Cape Town-based criminal advocate William Booth, who is not participating in the trial, said much will depend on how Pistorius holds up in that second phase.
"Oscar's evidence can only be properly assessed once he's been cross-examined," Booth said.
In the five weeks since the trial began, Pistorius has appeared fragile and sometimes annoyed, frequently crying in court.
He was physically sick when the gruesome details of Steenkamp's death were discussed.
His lawyer Barry Roux said he will call up to 17 witnesses in his case to testify on ballistics, urine emptying, damage to the toilet door, sound, and "disability and vulnerability".
The trial is slated to run to at least mid-May.