A tearful Oscar Pistorius apologised to Reeva Steenkamp's family, telling the court he had "terrible nightmares" since the night he killed his girlfriend and often woke up smelling blood.
Stammering and with his jaw trembling as he took the witness stand in his defence, Pistorius said he did not intend to kill his 29-year-old lover.
The Paralympic gold medallist is accused of premeditated murder, but claims he shot the model though a locked toilet door believing she was an intruder.
"I want to take this opportunity to apologise to Mr and Mrs Steenkamp," he said, at times barely audible as he sobbed.
"I was trying to protect Reeva, I want people to know that she was loved when she went to bed that night."
In the tribunal Steenkamp's mother June, flanked by other relatives, stared at him stony-faced during the apology.
She has for weeks sat through gruesome evidence, including brutal images of her dead daughter.
"I tried to put my words on paper to write to you but no words can ever suffice," Pistorius continued.
The athlete cut a broken and emotional figure as he spoke publicly for the first time about events on the fateful Valentine's Day Steenkamp died.
"I've had terrible nightmares," he said since the night of the shooting, adding: "I wake up at night smelling blood."
He also said that since the shooting he has taken sleeping pills and anti-depressants.
Pistorius told the court that on one occasion after waking in the middle of the night he crawled into a cupboard because he was so scared.
After five weeks of prosecution testimony, Monday was the first day of Pistorius's defence.
The court broke early for the day after defence lawyer Barry Roux said his witness was exhausted by a lack of sleep and the day's ordeal.
"Well he does look exhausted, he sounds exhausted," said Judge Thokozile Masipa, granting the defence an adjournment until Tuesday.
- Security concerns -
Pistorius used his testimony to outline his problems since the shooting and his early life growing up disabled and without his father present.
"My mother had a lot of security concerns, we grew up in a family where my father wasn't around much, she would often get scared at night, she would phone the police."
"My mother had a pistol," he added. "She kept her firearm in a padded bag under her pillow."
Sheila Pistorius died when her amputee son was aged 15.
"When she passed away it was very unexpected," he told the court. "We weren't informed that she was sick and by the time we were she was in a coma."
He also testified about his vulnerability when he doesn't wear prosthetics.
"I don't have balance on my stumps," he said.
After a lunch break the defence spent time on a character sketch of a deeply religious man whose exposure to crime and violence in the past created in him a fear for burglars at his house.
"When I grew up we were exposed to crime. Housebreakings, family members being assaulted and high-jacked," said the athlete, and recounted burglaries at his own and his father's houses.
Pistorius also testified about helping another man once attacked by people on the street, and spoke about his strong faith.
"I put a lot of faith in the Lord to be where I wanted to be," he said about a revival in faith in 2012, and described meeting the equally-religious Steenkamp as a "blessing".
"We would pray at night," he said.
- Gruesome details -
In the five weeks since the trial began, Pistorius has appeared fragile and annoyed, frequently crying in court and being physically sick when the gruesome details of Steenkamp's death were discussed.
He has said little since his "not guilty" plea besides the occasional "yes, milady" to Judge Thokozile Masipa.
How the double amputee known as the "Blade Runner" will hold up during the state's cross-examination, which could begin on Tuesday, may prove a turning point in the case.
Pistorius has hired an extensive team of forensic experts to describe the events in the early hours of February 14, 2013, including an American animation firm that will visually depict the crime scene using three-dimensional computer generated images.
The experts will have to cast doubt on the state's version of events, including testimony from witnesses who said they heard a woman screaming on the night of the murder, which would show Pistorius knew his target was Steenkamp.
His lawyer Barry Roux said he will call 14 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.