Oscar Pistorius waits for the start of his trial at the high court in Pretoria, South Africa, on Monday. Herman Verwey / Associated Press

South African Paralympian Oscar Pistorius pleaded not guilty on Monday to killing his girlfriend as his trial opened with a witness testifying she heard "bloodcurdling" screams on the night the woman died.

Pistorius, whose trial is being broadcast around the world, faces a life sentence if convicted of the Valentine's Day killing of model and reality television star Reeva Steenkamp a year ago.

After a state prosecutor charged that Pistorius "unlawfully and intentionally did kill" Steenkamp, Pistorius entered his "not guilty" plea.

Pistorius, 27, a double-amputee sports hero known as the "Blade Runner" for his carbon-fiber running blades, also pleaded not guilty to three unrelated gun charges.

In a statement read by his lawyer, Kenny Oldwage, he described Steenkamp's death as a "tragic accident" saying he had mistaken her for an intruder.

He admitted killing the woman, but denied murderous intent. "This allegation is denied in the strongest terms," he said. "We were in a loving relationship."

A single judge, Thokozile Masipa, is presiding in the case that has already been likened to the murder trial of former US football star OJ Simpson, who was accused in 1995 of killing his ex-wife and a friend.

The first prosecution witness to take the stand, Michelle Burger, said she was awakened at around 3 am by screams from the Pistorius home in an upmarket Johannesburg gated community.

"It was very traumatic," Burger said, speaking in Afrikaans through an interpreter. "You could hear it was bloodcurdling screams. You can't translate it into words. The anxiousness in her voice, and fear. It leaves you cold."

"She screamed terribly and she yelled for help," Burger added, testifying that after the gunshots a man shouted for help.

As the trial got underway, the victim's black-clad mother, June, came face-to-face with Pistorius for the first time since the killing.

"I want to look at Oscar, really look him in the eyes, and see for myself the truth about what he did to Reeva," she said ahead of the trial.

Farther down from her on the wood-paneled bench sat Pistorius' brother Carl, his sister Aimee and a handful of other relatives.

The state is seeking to prove that Pistorius killed Steenkamp in a rage after the couple quarreled in the early hours of Feb 14, 2013.

Prosecutors are also expected to claim that Pistorius had watched pornography just before the shooting, apparently contradicting his account of events.

Pistorius, who at the London 2012 Olympics was the first double-amputee to compete against able-bodied athletes, will also be asked whether he told security guards at his luxury estate that everything was fine when they phoned after hearing gunshots.

In a bid to illustrate a history of reckless behavior with firearms, the state claims that Pistorius on two occasions fired a pistol in public, once through the sunroof of a moving car and then at a busy restaurant.

AFP-AP-Reuters