When asked how he would plead to the charges of the "unlawful and intentional killing" of 29-year-old model Reeva Steenkamp on Valentine's Day last year, a sombre-looking Pistorius told judge Thokozile Masipa: "Not guilty, my lady."
Dressed in a dark suit, white shirt and black tie, the Paralympic and Olympic runner also pleaded not guilty to several other firearms charges, including once discharging a pistol under the table of a swanky Johannesburg restaurant.
The 27-year-old, who was born without legs but reached the 2012 Olympic 400 metres semi-final running on carbon-fibre "blades", argues that the killing of law graduate Steenkamp was a tragic case of mistaken identity.
He says he mistook her for an intruder hiding in the upstairs toilet of his luxury Pretoria home. As Pistorius entered the packed courtroom on Monday, Steenkamp's mother June followed him with her gaze.
Prosecutors will seek to prove that Pistorius fired four rounds from a 9 mm pistol through the door of the toilet in a deliberate attempt to kill whoever was behind it.
Steenkamp, a women's rights campaigner and regular on South Africa's celebrity party scene, was hit three times, in the head, arm and hip. She was declared dead at the scene.
If the state succeeds in convincing Masipa of intent to kill, Pistorius could go to prison for life, in all likelihood a minimum of 25 years behind bars.
At his bail hearing last year, Pistorius admitted to culpable homicide, equivalent to manslaughter, a crime that could see him put away for 15 years - or he could leave the Pretoria High Court a free man, with no more than a slap on the wrist and a suspended sentence.
Coming less than a month after the rape, disembowelling and murder of a teenager near Cape Town, the 2013 shooting of Steenkamp caused outrage and drew further attention to the high levels of violence against women in South Africa.
The trial before Masipa - juries were abolished by the apartheid government in the 1960s - is set to last a minimum of three weeks but with as many as 107 witnesses waiting to be called by either side it is almost certain to last far longer.
The proceedings have attracted massive media attention, with hundreds of foreign and domestic media camped outside the Pretoria court.
The trial is being broadcast live on television, putting the South African justice system under intense scrutiny.
Two decades after the end of apartheid, many South Africans feel that wealthy defendants such as Pistorius are able to win a better quality of justice than the majority as they can afford to hire the best lawyers and experts.
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- Reeva Steenkamp