The first witness at Oscar Pistorius' murder trial told the court she heard "bloodcurdling screams" from a woman followed by shots.

It was a dramatic opening to a case that could see one of global sport's most admired role models jailed for life.

Taking the stand at Pretoria High Court after the 27-year-old Para­lympic and Olympic running star pled not guilty to murdering his girlfriend, model Reeva Steenkamp, on Valentine's Day last year, neighbour Michelle Burger testified she was woken in the middle of the night by a woman shouting for help.

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"I was still sitting in the bed and I heard her screams," said Ms Burger, who lives 194 yards from Pistorius's home in an adjacent housing complex.

"She screamed terribly and she yelled for help. Then I also heard a man screaming for help. Three times he yelled for help," she said, speaking in Afrikaans through an interpreter.

Thinking it was a violent break-in - a possibility in crime-ridden South Africa - Ms Burger said her husband called the private security firm guarding their upmarket Pretoria housing estate before the pair heard more shouts.

"I heard the screams again. It was worse. It was more intense," said Ms Burger, a Pretoria University economics lecturer. "She was very scared," she added.

"Just after her screams, I heard four shots. Four gunshots. Bang … bang, bang, bang."

"It was very traumatic for me. You could hear that it was bloodcurdling screams."

After the final shot, the screams started fading, she added later.

Throughout Ms Burger's testimony, Pistorius - described by Time magazine in 2012 as "the definition of global inspiration" and named as one of the world's 100 most influential people - sat impassively in the courtroom, staring at the floor.

The athlete, who was born without lower legs but reached the 2012 Olympic 400 metres semi-final using carbon-fibre "blades", argues Ms Steenkamp's killing was a tragic accident after he mistook her for an intruder hiding in the toilet.

Ms Burger maintained her testimony despite cross examination by lead defence advocate Barry Roux.

The proceedings have attracted massive media attention, with hundreds of foreign and domestic media outside the court. The trial is also being broadcast live, a first for South Africa

Before Ms Burger was called as a witness, Pistorius, dressed in a dark suit, white shirt and black tie, stood before Judge Thokozile Masipa to plead not guilty to murdering law graduate Ms Steenkamp, a women's rights campaigner and familiar face on South Africa's celebrity party scene.

He also pled not guilty to several other firearms charges, including one of discharging a pistol under the table of a posh Johannesburg restaurant and another of putting a bullet through the sun-roof of a former girlfriend's car.

When he entered the packed courtroom, Ms Steenkamp's mother June followed him with her gaze. Her father Barry was not in court after recently suffering a stroke.

Prosecutors are seeking to prove Pistorius fired four rounds from a 9mm pistol through the door of the toilet in a deliberate attempt to kill whoever was behind it.

Ms Steenkamp was hit three times, in the head, arm and hip. She was declared dead at the scene.

In his opening address, lawyer Kenny Oldwage, who with Mr Roux forms part of Pistorius's defence team, sought to portray the state's allegations as an unwarranted character assassination of a young man deeply in love.

If the state succeeds in convincing Judge Masipa of intent to kill, Pistorius could get life and a minimum of 25 years behind bars.

At his bail hearing last year, he admitted to culpable homicide, equivalent to manslaughter, which could see him put away for 15 years - or he could leave court a free man, with little more than a suspended sentence.

The trial before Judge Masipa - juries were abolished by the apartheid government in the 1960s - is due to last a minimum of three weeks, but with as many as 107 witnesses waiting to be called by either side it is likely to last far longer.

After the hearing, Pistorius left the court through a scrum of photographers and television cameras before being bundled into a waiting car.

The trial continues.