Michell Burger, a woman who lives on an estate next to Pistorius' gated community, said she and her husband were awoken by the screams in the pre-dawn hours of Feb. 14 last year, when Pistorius killed Reeva Steenkamp by shooting four times through a door in his bathroom.
Pistorius says he killed Steenkamp by mistake thinking she was a dangerous intruder in his house, but prosecutors believe the world-famous athlete shot his girlfriend after a fight and immediately tried to paint a picture at the trial of a loud argument before the fatal shots.
"It was very traumatic," Burger said, speaking in Afrikaans through an interpreter and in answer to questions from lead prosecutor Gerrie Nel. "You could hear it was blood-curdling screams. You can't translate it into words. The anxiousness in her voice, and fear. It leaves you cold."
Burger said: "She screamed terribly and she yelled for help" and testified that after the gunshots a man shouted for help.
Pistorius earlier pleaded not guilty at the start of the trial to charges of murder and three other counts relating to shooting guns in public and illegal possession of ammunition. Wearing a dark gray suit and black tie, he spent the first 30 minutes of the first day of the trial at the high court in the South African capital standing before his defense lawyer, Barry Roux, asked the judge for permission for Pistorius to sit.
The Olympian's murder trial was being broadcast live on TV in South Africa and across the world.
The first witness, Burger, was called before even an hour had passed as the prosecution went straight into testimony. The trial itself started 90 minutes late after an earlier delay because of an interpreter problem.
When he entered court, Pistorius walked past the victim's mother who says she came to court so she can "really look him in the eyes."
Defense lawyer Kenny Oldwadge laid out Pistorius' legal strategy, reading a statement from Pistorius in which he says the killing was an accident and that there were inconsistencies in the state's case, as well as an attempt to introduce inadmissible character evidence to discredit him.
In the statement, Pistorius said he brought two fans in from the balcony on the night of the killing, after speaking to his girlfriend who was in bed beside him. He said Steenkamp must have gone into the bathroom while he was fetching the fans. Pistorius said he did not notice that she had gone and heard the bathroom window open.
"I approached the bathroom, armed with my firearm, so as to defend Reeva and I," Pistorius said in the statement. He said he then heard a noise in the toilet, and was in a "fearful state" because he was unable to run away or defend himself physically since he was not wearing his prosthetic legs.
"The state has embarked on a strategy to rely on unsubstantiated allegations," he said, citing a neighbor's testimony that an argument had been heard in his home.
According to Pistorius' statement, other neighbors living nearby said they had not heard any argument. He also cited evidence provided by police detective Hilton Botha as "false in material respects."
In the courtroom, Pistorius was seated near Steenkamp's mother, June. She was quoted in the Pretoria News, which published an interview she gave to a British newspaper, saying that she wants to see Pistorius.
"I want to look at Oscar, really look him in the eyes, and see for myself the truth about what he did to Reeva," said June Steenkamp, 67. "Whatever the court decides at the end of his trial, I will be ready to forgive him ... But first I want to force him to look at me, Reeva's mother, and see the pain and anguish he has inflicted on me. I feel I need that."
Reeva Steenkamp was a glamorous model and budding reality TV show star when she was cut down at age 29.
Earlier, a drone carrying cameras flew over the entrance to the courthouse in gray, drizzly skies. Several broadcasters massed live broadcast satellite trucks around the courthouse. A 24-hour cable channel devoted to covering the trial was launched in South Africa on Sunday.
If convicted on the murder charge, Pistorius could be sent to prison for at least 25 years before the chance of parole, the minimum time someone must serve if given a life sentence in South Africa. South Africa does not have the death penalty.
A lesser sentence is possible if Pistorius is found guilty of murder but without premeditation. He also could be convicted of culpable homicide, South Africa's version of manslaughter in which someone is killed through negligence.
The additional firearms charges relate to him allegedly shooting out of the sunroof of a car in one incident and another when he allegedly fired a gun inside a restaurant, apparently by mistake. Ammunition also was found at his house after the shooting that he allegedly did not have proper licensing for.
Judge Thokozile Masipa, hearing the biggest trial of her career, will ultimately pronounce the champion runner innocent or guilty and will decide on any sentence. South Africa has no trial by jury.
Parts of the trial will be broadcast on live television, both in South Africa and across the world. A South African cable channel has been launched which will provide 24-hour coverage of the Pistorius trial, using commentators and prepared feature stories when the court is not in session.