A South African prosecutor has accused Oscar Pistorius of lying and altering his story when the Olympic and Paralympic athlete described the night he shot dead his girlfriend Reeva Steenkamp on Valentine's Day last year.             

The double amputee sprinter faces life in prison if convicted in the Pretoria High Court of the murder of Ms Steenkamp, a 29-year-old law graduate and model.             

Mr Pistorius, known as 'Blade Runner' due to the prosthetics he wears on the track, says he shot Ms Steenkamp in a tragic accident, firing at what he thought was an intruder hiding behind a toilet door in his luxury Pretoria home on 14 February last year.

Prosecutor Gerrie Nel cross-examined Mr Pistorius while looking through photographs of the couple's bedroom taken after the shooting.

Mr Nel pointed out a series of objects in one picture which ran counter to Mr Pistorius' account of events.

In response, the sprinter accused the police of tampering with the scene, but Mr Nel ridiculed this suggestion.

"Let's sum up: A policeman moved the two fans, put the duvet on the floor, opened the curtains wider than they should be before the photographs were taken," Mr Nel said.

"Your version is so improbable that nobody would ever think it's reasonably possibly true ... Your version is a lie."

Mr Pistorius says after he shot Ms Steenkamp, he ran out onto the balcony and screamed for help.

However, in a picture shown to the court, a duvet and a large stand-up electrical fan clearly block his route to the balcony door.

With no direct witnesses, Mr Nel's main task is to pick holes in Mr Pistorius' testimony and cast doubt on his assertion he believed a burglar was in his house, a common fear in crime-obsessed South Africa.

Witnesses in nearby buildings have testified to hearing a woman's scream before the sound of shots, which the prosecution hopes will help prove that the couple had a heated argument before Mr Pistorius intentionally killed Ms Steenkamp.

During much of the 19-day trial, Mr Nel has sought to portray Mr Pistorius as an arrogant temper-prone man who is reckless with firearms and refuses to take responsibility for his actions.

"You will blame anybody but yourself," Mr Nel said to the 27-year-old track star, cross-examining him about a separate incident in which Mr Pistorius is accused of firing a pistol in a packed restaurant.

Mr Pistorius said the gun was given to him by a friend under the restaurant table and went off by itself.

Police Captain Christian Mangena gave evidence earlier in the trial, saying the weapon could only fire if the trigger was pulled.

The athlete said he could not explain how the gun went off and questioned his own defence advocate Barry Roux's decision not to cross-examine Mr Mangena on his evidence.

"Now you blame counsel Mr Roux," Mr Nel said, prompting Mr Roux to shake his head at a colleague.

"You are lying," Mr Nel said, holding Pistorius in a stare. "You just refuse to take responsibility for anything."

Mr Pistorius earlier broke down and sobbed as Mr Nel pushed him repeatedly to take responsibility for killing Ms Steenkamp.     

He denied "picking on" her. He was commenting on a mobile phone message Ms Steenkamp had sent him.

Mr Nel read from one message which said: "I've been upset with you for two days now, I'm scared of you sometimes."

He added: "Why would she be scared of you?"

Mr Pistorius replied: "I think she's scared of the feelings that she had for me, she says 'I'm scared of you sometimes and how you snap at me and how you will react to me'.

"I never shouted or screamed at her. It hurt her feelings about the way I would react."

Mr Nel accused the athlete of "picking on" Ms Steenkamp, including an argument after he moved away from her when she began touching his neck in public.

He said the phrase "I love you" never appeared in either of their phone messages to each other.

But Mr Pistorius insisted his relationship was open and trusting with Ms Steenkamp and said he did not write text messages showing his feelings because he preferred to talk to her.

In a dramatic opening to his cross-examination yesterday, Mr Nel shocked the Pretoria court when he confronted Mr Pistorius with a graphic photograph of the dead Ms Steenkamp showing the side and back of her skull, her hair matted with blood and brains.

Mr Pistorius broke down and sobbed as Mr Nel pushed him repeatedly to take responsibility for killing Ms Steenkamp.