Reeva Steenkamp," the 26-year-old said in an affidavit read out at a court hearing in Pretoria, his first public comments since the February 14 killing.
Magistrate Desmond Nair hinted strongly that the double-amputee Olympian and Paralympian "Blade Runner" would not be released on bail and could face a charge of premeditated murder.
But Pistorius said he fired at the door of the bathroom where his 29-year-old lover was hiding as he was "filled with horrible fear" that someone had sneaked into his luxury Pretoria home through an open window.
"I fired shots at the door and shouted at the toilet," he said in the affidavit. "We were deeply in love and couldn't be more happy."
Prosecutor Gerrie Nel had told the court Pistorius had armed himself, put on his prosthetic legs, walked seven metres and fired four shots through the locked bathroom door, hitting a terrified Steenkamp three times and fatally wounding her.
Olympian Oscar Pistorius stands following his bail hearing in Pretoria, South Africa. Pistorius fired into the door of a small bathroom where his girlfriend was cowering after a shouting match on Valentine's Day, hitting her three times, a South African prosecutor said Tuesday as he charged the sports icon with premeditated murder. AP Photo
"She could go nowhere," he said, adding that Pistorius had "shot and killed an unarmed innocent woman".
Pistorius broke down in court on Tuesday as he was accused of shooting his model girlfriend three times through a locked bathroomdoor in what prosecutors said was a "premeditated" Valentine's Day murder.
The double-amputee Olympian and Paralympian "Blade Runner" is seeking bail at the hearing which is expected to reveal more about what happened the night his covergirl law graduate lover Reeva Steenkamp was killed.
The 26-year-old defendant, making his second court appearance over the killing, appeared in the dock in a black suit, blue shirt and grey tie and stared straight ahead anxiously, sometimes mumbling under his breath.
Steenkamp, 29, who was shot four times in the early hours of Thursday by a 9mm pistol owned by the sprinter, is due to be laid to rest at a private ceremony in her hometown of Port Elizabeth later on Tuesday.
Pistorius's family has insisted that the evidence will refute "any possibility of a premeditated murder or indeed any murder at all".
Defence lawyers are expected to argue that there are "exceptional circumstances" for Pistorius to be freed pending trial.
As a result, more details will likely emerge of what happened during the Valentine's Day shooting at Pistorius's upmarket home in a gated estate in Pretoria.
We were deeply in love: Pistorius on girlfriend | Excerpts of prosecutor's case against Oscar Pistorius
The state has said it would oppose bail.
Both sides have called in heavyweight lawyers, setting the scene for a massive legal showdown.
Unconfirmed reports have suggested Pistorius mistook Steenkamp for an intruder, while other reports said she was beaten with a cricket bat before being shot multiple times.
In an interview published on Monday, Steenkamp's mother described Reeva's death as "horrendous."
"Why my little girl? Why did this happen? Why did he do this?" June Steenkamp told the Times of South Africa.
"She had so much of herself to give and now all that is gone. Just like that, she is gone... In the blink of an eye and a single breath, the most beautiful person who ever lived is no longer here."
Steenkamp, who had been going out with Pistorius since late last year, will be laid to rest in a private ceremony at a Port Elizabeth crematorium with an on-site chapel that holds less than 100 mourners.
Reeva's cloth-draped coffin bearing white flowers was carried into the chapel by six people.
It arrived in a black hearse.
A few mourners have arrived for the ceremony.
"The memorial itself inside the chapel is family and friends and then there will be a statement afterwards for the media," her uncle Michael Steenkamp said.
The family bore the runner no grudge but wanted clarity about Steenkamp's death.
"The family haven't got that animosity or hatred or anything like that but questions, and we realise that it's going to come out," he said.
Sharon Steenkamp, her cousin, told local newspaper The Herald that none of the family at Steenkamp's parents home had met Pistorius.
"No, we are not even thinking about Oscar. All our focus is on Reeva, her life and what has happened to her. She never had an enemy in the world and was a super person," she said.
On Saturday a celebrity television show aired haunting footage of Reeva Steenkamp speaking about the need to leave a positive mark on life, words laden with poignancy after her death.
"Not just your journey in life, but the way that you go out and make your exit is so important, you have either made an impact in a positive way or a negative way," she said.
Pistorius, who broke down in sobs during his first court appearance on Friday, has built up a powerful team of lawyers, medical specialists and public relations experts for his defence.
Stuart Higgins, a former editor of British tabloid The Sun, whose lengthy list of clients includes British Airways, Chelsea FC and Manchester United football club, has taken over his public relations.
One of the lawyers, Kenny Oldwage, defended the driver in a 2010 accident that killed former president Nelson Mandela's great-grandchild Zenani. The driver was acquitted.
A global inspirational figure, Pistorius was the first double amputee to compete against able-bodied athletes in the 2012 London Olympic Games. His nickname was inspired by his custom-built carbon-fibre running blades.
The Johannesburg-born runner had both legs amputated below the knee when he was 11 months old after being born without lower leg bones.
The killing has put his career on hold, forcing him to cancel races in Australia, Brazil, Britain and the United States between March and May.