Oscar Pistorius has pleaded not guilty to a charge of murder at the start of his trial at Pretoria High Court in South Africa.
"Not guilty, My Lady," Mr Pistorius answered when asked by Judge Thokozile Masipa.
He also pleaded not guilty to three firearms charges.
Mr Pistorius was charged with contravening South Africa’s firearms laws after he shot through the sunroof of the car of his former lover.
He also stands accused of recklessly discharging a firearm at a restaurant table in a manner that could have endangered other patrons.
The third charge relates to possession of ammunition without a permit.
The Paralympic and Olympic athlete shot and killed his girlfriend, model Reeva Steenkamp, on Valentine's Day last year.
The 27-year-old argues that it was a tragic case of mistaken identity and that he mistook Ms Steenkamp for an intruder hiding in the toilet.
Court hears of 'bloodcurdling' screams at Pistorius' house
The first witness at the trial told the court she heard "bloodcurdling" screams from a woman followed by four shots.
Michelle Burger, who lived in a nearby estate, told of being woken in the middle of the night by a woman screaming and shouting for help.
"I was still sitting in the bed and I heard her screams," Ms Burger said, speaking in Afrikaans through an interpreter.
"She screamed terribly and she yelled for help. Then I also heard a man screaming for help. Three times he yelled for help."
"Just after her screams, I heard four shots. Four gunshots ... 'Bang ... bang, bang, bang'," she said, her voice cracking with emotion.
At Ms Burger's request, her testimony was not shown on television, but the audio was broadcast live.
"It was very traumatic for me. You could hear that it was bloodcurdling screams," she added.
Pistorius accused of premeditated murder
Mr Pistorius is accused of the premeditated murder of Ms Steenkamp, who died of multiple gunshot wounds.
Prosecutors allege the paralympian shot the model and reality TV star four times through the locked bathroom door at his home in Pretoria in the early hours of 14 February last year.
Some witnesses have described hearing a woman scream, followed by "moments of silence", then hearing gunshots followed by more screaming.
Mr Pistorius claims he thought an intruder was in his home, but prosecutors say that even so, an "error in persona" did not affect the fact he intended to kill someone.
Some 107 witnesses are listed by the state, including Ms Steenkamp's close friends Desiree, Gina and Kim Myers, as well as Mr Pistorius's sister Aimee and his uncle Arnold.
Ms Steenkamp's mother June is attending the trial. Her family have said they were looking for "closure".
On the anniversary of Ms Steenkamp's death, Mr Pistorius made a statement on his website saying: "No words can adequately capture my feelings about the devastating accident that has caused such heartache for everyone who truly loved - and continues to love Reeva.
"The pain and sadness - especially for Reeva's parents, family and friends consumes me with sorrow.
"The loss of Reeva and the complete trauma of that day, I will carry with me for the rest of my life."
The mandatory sentence for someone convicted of premeditated murder in South Africa is life with a minimum of 25 years in prison, meaning if Mr Pistorius is found guilty, he will be over 50 when he leaves prison.
The trial has been adjourned until Tuesday morning, when the defence will continue to question Ms Burger.