Oscar Pistorius claimed he was the victim of "character assassination" in the opening statement of his trial for killing Reeva Steenkamp.

Pistorius stated "there was no argument" between him and Steenkamp on Valentine's Day last year - the night he shot her in the toilet of his luxury home in a suburb of Pretoria.

The disabled star went on the offensive, mounting an attack on the prosecution case in his opening statement at North Gauteng High Court in Pretoria.

The case has global media attention: Pistorius is one of the most recognisable athletes on the planet.

Prosecution lawyers intend to prove Pistorius blasted Steenkamp to death following an argument.

Pistorius, 27, said prosecution witnesses had given "contradictory statements" which were "unfair". He also said early investigations by police had left the crime scene tainted.

Pistorius claimed that "inferences are being drawn from unsupported facts" and that "inadmissible character evidence" had been "entered to engineer an assassination of my character".

Pistorius's strongly-worded rebuttal of the prosecution's case was delivered by his lawyer Barry Roux.

Earlier, prosecution lawyers had admitted there were no eye-witnesses to Steenkamp's death and said their case was built upon strong circumstantial evidence.

With Reeva Steenkamp's mother June watching him from only yards away, Pistorius said he and Reeva had been in a "loving relationship."

"The occurrence was an accident," said Roux for Pistorius. "I believe intruders posed an imminent threat to Reeva and me."

He continued: "There is no evidence I fired the shots because of the 'argument'. I deny this in the strongest terms because there was no argument."

Pistorius said the prosecution was wrong to claim he knew Steenkamp was in the toilet when he fired four times through the door.

Roux said Pistorius believed there were intruders in the bathroom because he did not know Steenkamp had gone to the toilet while he was fetching two fans from a balcony.

"Firing in my fearful state, knowing I was on my stumps, I believed there to be intruders coming to attack Reeva and me," said Roux for Pistorius.

"There is no basis for alleging I wanted to take Reeva Steenkamp's life. Evidence will corroborate my version of events."

The trial continues.