Updated March 3, 2014 11:34 a.m. ET  PRETORIA, South Africa—A neighbor to Oscar Pistorius told a jammed courtroom on the first day of his murder trial that she awoke to the sound of "bloodcurdling" screams coming from the former Olympian's house, followed by a man's shouts for help.  Michelle Burger said she assumed a burglary was under way next door when she heard the crack of four gunshots echo through the night. It wasn't until the next morning, she told the court, that she realized the screams belonged to Reeva Steenkamp, the 29-year-old model and reality-TV star whose murder on Valentine's Day last year Mr. Pistorius was formally charged with on Monday.  "The fear in that woman's voice is difficult to explain," Mrs. Burger said, adding that she sounded "absolutely petrified."  Mrs. Burger's testimony launched the prosecution's case against Mr. Pistorius and came after Mr. Pistorius pleaded not guilty to one count of premeditated murder and several counts of reckless use of firearms.  Mr. Pistorius, a double amputee who overcome his disability to become a national hero and international celebrity, has admitted firing four shots through a locked bathroom door at his home in the gated Silver Woods Estate, on the eastern outskirts Pretoria. He said, however, he mistook the person inside for an intruder--and denies intentionally killing his girlfriend. Ms. Steenkamp was hit by three bullets and died at the scene.  Dressed in a black suit and tie, he arrived at the packed courtroom in Pretoria and walked past the dead woman's relatives, including her mother, without a glance.  In his opening statement, Kenny Olwage, Mr. Pistorius's lawyer, denied that Mr. Pistorius and Ms. Steenkamp were arguing at the time the fatal shots were fired. The pair, Mr. Olwage said, were in a "loving relationship." By suggesting otherwise, state prosecutors were engaged in "character assassination," he added.  Mr. Pistorius' other lawyer, Barry Roux, sought to poke holes in Mrs. Burger's testimony in his cross examination.  Upon realizing it was Ms. Steenkamp in the bathroom, Mr. Pistorius called for help from the balcony and then banged against the locked bathroom door three times with a cricket bat until it broke, according to his lawyer.  Mr. Roux said that would have been loud enough to resemble gunshots. He also pointed to the conflicting statements between Mrs. Burger and her husband. Her husband said in a police statement that he heard five or six shots to the four Mrs. Burger heard.  He also tried to say that Mrs. Burger's testimony on Monday about the fear evident in the woman's screams was not reflected in her earlier statement to police.  Mrs. Burger, in reply, said she was "100%" certain what she heard were gunshots and not the sound of a cricket bat. She also said she couldn't testify about what her husband heard and that she had expressed the "anxiety" in the screams.  Mr. Pistorius sat attentively, taking notes during the witness questioning. His courtroom decorum marked a contrast to his continuous sobbing during the bail hearing last year. As he left court Monday, he refused to speak with reporters. His family also declined to comment.  Ms. Steenkamp's mother June sat staring at Mr. Pistorius or at the witness.  On the one-year anniversary of the shooting, Mr. Pistorius released a statement in which he said: "The loss of Reeva and the complete trauma of that day, I will carry with me for the rest of my life."  The 27-year-old Mr. Pistorius was born without fibulas in both legs, which led to the amputation of both limbs below the knee when he was a child.  Known as the "Blade Runner" for the carbon-fiber prosthetics he donned for competitions, he went on to become a record-breaking sprinter, win a gold medal at the 2012 Paralympic Games in London and compete that same year in the Olympics, also in London.  In addition to making him famous, his triumphs on the track earned Mr. Pistorius lucrative sponsorship and advertising deals with Nike Inc. NKE -1.16% Nike Inc. Cl B U.S.: NYSE $77.39 -0.91 -1.16% March 3, 2014 4:01 pm Volume (Delayed 15m) : 3.03M AFTER HOURS $77.39 0.00 0.00% March 3, 2014 4:15 pm Volume (Delayed 15m): 5,371 P/E Ratio 24.20 Market Cap $69.33 Billion Dividend Yield 1.24% Rev. per Employee $547,063 02/18/14 Captain Corporate: Jeter After... 02/07/14 Asics CEO Kevin Wulff on His L... 01/19/14 Major Brands Confront Cambodia... More quote details and news » NKE in Your Value Your Change Short position and Oakley sunglasses.  The prosecution faces the challenge of overcoming errors by the police in the initial days after the killing. The original police investigator was removed from the case when it was disclosed that he had failed to wear protective footwear at the crime scene and missed detecting a bullet that had fallen into the bloody toilet bowl.  Write to Devon Maylie at devon.maylie@wsj.com
The testimony came after former Olympian Oscar Pistorius entered a plea of not guilty on charges of murdering his girlfriend, model Reeva Steenkamp.