Pretoria, South Africa (CNN) -- An emotional Oscar Pistorius apologized Monday to the family of Reeva Steenkamp, the girlfriend he killed on Valentine's Day last year, saying he woke up thinking of them and praying for them every day.
"I would like to take this opportunity to apologize -- to Mr. and Mrs. Steenkamp, to Reeva's family -- to those who are here today who knew her," Pistorius said as he took the stand for the first time at his murder trial.
"I can't imagine the pain and the sorrow and the emptiness that I have caused you and your family. ... I can promise you that when she went to bed that night, she felt loved," he said, his voice breaking as if he was fighting back tears.
It was the first time he has spoken in public about Steenkamp's death, which he says was an accident. He pleaded not guilty to murder when the high-profile trial opened last month.
Steenkamp's mother, June, sat stony-faced in court as South Africa's onetime Olympic golden boy choked out his statement.
Judge Thokozile Masipa also betrayed no emotion as Pistorius spoke but did once ask him to talk louder, saying she could hardly hear him.
Monday was the first day of the defense phase of the trial, following three weeks of prosecution in March.
Pistorius, who says he mistook Steenkamp for an intruder in his house in the dark, testified that he has been suffering nightmares since the killing and wakes up smelling blood.
He told the Pretoria court that he is afraid to sleep, and "if I hear noise, I wake up just in a complete state of terror." He said he is on medication, including an antidepressant and sleeping aids.
Earlier, the first defense witness, pathologist Jan Botha, talked about the wounds Steenkamp suffered when Pistorius shot her and about when she last ate.
The South African amputee sprinter put his head in his hands as Botha said that the shot that hit Steenkamp's arm was "akin to a traumatic amputation" and that she died "fairly quickly after sustaining the head wound."
Botha went first because of "family health reasons," Roux said, breaking with the South African legal custom of the defendant testifying first. He said Botha was the only defense witness who will go "out of order."
Pistorius, 27, is accused of intentionally murdering Steenkamp, 29.
The defense team will call 14 to 17 witnesses, Barry Roux, Pistorius' lead lawyer, said as he opened his case.
The prosecution rested its case on March 25 after 15 days and 21 witnesses.
Pistorius took the stand late Monday morning after the pathologist finished his testimony.
Roux spent much of the day trying to build up a picture of Pistorius as a dedicated athlete, responsible person and devoted Christian who was "bowled over" by his love for Steenkamp.
Pistorius was also prompted to talk about his awareness of crime, including having come to the aid of victims of crime.
Talking about his childhood, Pistorius said his mother kept a firearm in a padded bag under her pillow. His father was often not around, and Pistorius said his mother would sometimes wake her children up, thinking they were being burgled.
He said she was very supportive of him and "never made me feel any different from the rest of the kids."
"Everything I learned in life, I learned from her," he said.
He spoke about her death when he was 15 and attending boarding school. He did not know she was sick until he got a call asking him to come visit her at a hospital, he said.
Later, he got a call from doctors telling him to come immediately and arrived when she was on her deathbed. She died 10 minutes after he arrived, Pistorius testified.
She had encouraged him to be a normal child and participate in sports despite his disability, he said.
Roux took him through his athletic triumphs, including his success as a Paralympic sprinter, but also highlighted times he felt vulnerable or afraid.
He was badly injured in a boating accident in 2009, he said, which left him "a lot more vigilant about losing my life ... more fearful."
And he said he cannot stand still without his prosthetics on.
"I don't have balance on my stumps," he said. "I can't stand still on my stumps."
That could be a key to his defense. He says he fired his gun because he would have been unable to defend himself or run away when he heard what he thought was a burglar.
Roux asked for court to adjourn for the day about 20 minutes early after Pistorius testified that he did not sleep the previous night.
Prosecutor Gerrie Nel seemed to be on the verge of objecting when Masipa intervened, saying, "He does look exhausted."
The trial is due to resume Tuesday morning.
Trial to last until mid-May
Pistorius admits that he killed Steenkamp, firing four shots through a closed door in his house in the early hours of February 14, 2013. Three hit her, with the last one probably killing her almost instantly, according to the pathologist who performed the autopsy.
But Pistorius says he thought she was a nighttime intruder in his pitch-black house and believed he was firing in self-defense.
The trial, which began on March 3, is scheduled to continue until the middle of May.
Pistorius first achieved global fame as an outstanding double-amputee sprinter who ran with special prostheses that earned him the nickname "Blade Runner."
Masipa will decide the verdict in collaboration with two experts called assessors. South Africa does not have jury trials.
In South Africa, premeditated murder carries a mandatory life sentence with a minimum of 25 years in prison. Pistorius also could get five years for each of two unrelated gun indictments and 15 years for a firearms charge he also faces.
If he isn't convicted of premeditated murder, the sprinter could face a lesser charge of culpable homicide, a crime based on negligence.
The sentence for culpable homicide is at the judge's discretion.
CNN's Richard Allen Greene reported and wrote from London; Nicola Goulding reported from Pretoria, South Africa; and Emily Smith reported from Atlanta. CNN's Marie-Louise Gumuchian and legal analyst Kelly Phelps contributed to this report.