The first witness in the murder trial of Olympic runner Oscar Pistorius has told the court she heard the "blood-curdling" screams of a woman followed by four gunshots.

Pistorius' neighbor Michell Burger told the court that in the early morning hours of February 14 last year she and her husband were awakened by the screams of a woman shouting for help.

She said there was a second set of screams, and then a series of gunshots.

Pistorius has admitted to shooting model Reeva Steenkamp, but said he mistakenly believed he was firing at an intruder.

Burger's testimony contradicts the Olympic athlete who says he believed Steenkamp was in bed at the time he fired the shots.

Pistorius told the judge Monday he was not guilty of murder, recklessly discharging a firearm or illegally possessing ammunition.

Prosecutors say Pistorius killed Steenkamp after an argument that night.

If convicted of premeditated murder, Pistorius faces a life sentence.

Families of Pistorius and Steenkamp were also in court Monday, seated a few meters apart on opposite ends of a long bench.

The much-anticipated case has generated enormous media attention in South Africa and elsewhere. Parts of the trial are being broadcast live, and a South African station has launched a 24-hour channel dedicated to the proceedings.

The 27-year-old Pistorius is known as the "Blade Runner" for his carbon-fiber prosthetic legs. He competed in the 2012 London Olympics.