Pistorius retches as he details moments before shooting

South African Olympic and Paralympic sprinter Oscar Pistorius broke down and sobbed in the witness stand on Tuesday as he described the moments when he shot dead his girlfriend, Reeva Steenkamp, thinking she was an intruder hiding in the toilet.

The double amputee, who is accused of murdering 29-year-old law graduate and model Steenkamp, told a Pretoria court how he heard a window sliding open in his bathroom in the middle of the night on Valentine's Day last year.

He said he was convinced an intruder was breaking in and that he needed to arm himself.

"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."

Grabbing a 9 mm pistol lying under the bed, he moved on his stumps down the passageway leading from the bedroom towards the bathroom and the would-be intruders, he testified.

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According to his account, as he peered round the door of the bathroom, his right arm holding the 9 mm outstretched, his left steadying himself against the wall, he noticed the bathroom window was open, confirming his worst fears.

"I wasn't sure where to point the firearm," he said. "My eyes were going between the windows and the toilet. I stood for some time."

"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."

He said he continued to shout at Steenkamp to call the police, but slowly started to realise that she was not responding and might have been the one behind the door.

As he described to the court bashing in a door panel to gain access to the toilet, he broke down, sobbing uncontrollably in the witness stand, leading judge Thokozile Masipa to adjourn the hearing for the day.

If found guilty of murder, he faces life in prison.