The former neighbor of Oscar Pistorius testified at the athlete's murder trial on Tuesday, giving evidence on the night he shot his girlfriend dead, saying he heard a man crying loudly and that he called the security of the housing estate for help.

Michael Nhlengethwa was a witness summoned by the Olympian's defense team, which contends that Pistorius killed Reeva Steenkamp by mistake after believing there was an intruder in his home on Feb 14, 2013. The prosecution says Pistorius intentionally killed Steenkamp after the couple had an argument.

The disabled runner shot four times through a toilet cubicle door with his 9 mm pistol, hitting Steenkamp in the hip, arm and head.

Security checks at the courthouse where Pistorius is on trial were disrupted early on Tuesday because of what police said was a protest over work conditions by security staff, who left their posts and gathered outside the building. Some people entered the courthouse without having their bags checked, and Pistorius was heard asking whether a security check had been conducted as he entered the courtroom.

Nhlengethwa, who was Pistorius' next-door neighbor, testified that his wife woke him up after hearing a bang and that he did not hear gunshots. He said he then heard a man crying in a high-pitched voice that indicated he urgently needed help. Nhlengethwa could not make out most of what the man was saying, though he heard the words: "No, please, please, no."

The neighbor's bedroom window was around 25 meters from the balcony doors of Pistorius' bedroom, closer than neighbors called by prosecutors and who said they heard a woman screaming on the night of the shooting.

The defense was trying to present Nhlengethwa as a more reliable witness on the events of the night because of how much closer he lived to Pistorius' villa. A neighbor who testified to hearing a woman's "blood-curdling" screams on the night lived around 170 meters from Pistorius' house.

Nhlengethwa testified that he called estate security to ask them to come to their road.

During the testimony, Pistorius sat on a wooden bench in the courtroom, looking impassively at the witness.

Nhlengethwa, who said he runs a civil engineering company, earlier described how Pistorius was the first neighbor to welcome him to the gated community when he moved there in late 2009.

"He was basically that friendly neighbor," Nhlengethwa said. He said he also met Steenkamp once on the weekend before she was killed by Pistorius, and was struck by the warmth of her personality.

"I don't think I will ever forget that moment," he said. "She just opened her arms. She just came and hugged me."