The Olympian, 27, is charged with premeditated murder in Steenkamp's death and faces a life sentence with a minimum of 25 years before parole if convicted on that charge.
On the witness stand, he began to cry loudly while testifying, forcing the judge to rule a brief adjournment. Pistorius didn't stand up when the judge left, and also started to wail as he sat slumped over on the witness stand, his head in his hands. His brother and sister went over to him in an apparent attempt to comfort him and after a while he left the courtroom through a side door, still crying.
When Judge Thokozile Masipa returned, she called an early adjournment. Pistorius had by that time had returned to sit, jaw clenched, in the witness box. Court was to reconvene on Wednesday.
Pistorius pleaded not guilty to murder at the start of his trial and denied in earlier testimony Tuesday three other charges against him relating to firing a gun in public on two occasions, and illegal possession of ammunition. On one of the counts two witnesses have testified that Pistorius recklessly shot a gun out of a moving car in September 2012, months before he killed Steenkamp. Pistorius said: "It never happened."
Led by defense lawyer Barry Roux for the second day of his testimony, Pistorius also said he wasn't to blame for a shot going off in a busy Johannesburg restaurant because a friend handed him an "unsafe" gun with a bullet in the chamber under the table, Pistorius said. That happened weeks before he fired through the door in his home to hit Steenkamp in the head, arm and hip.
Pistorius also said he wasn't guilty of illegally possessing .38-caliber ammunition in his home because he was safekeeping it for his father and he had no intention to use it.
In a dramatic day, Pistorius had also left the room briefly at one point to change out of his dark suit and into a white shirt and shorts, with his prosthetic legs showing. He was then asked by Roux to take off his prosthetics in court and stand on his stumps by the bullet-marked toilet door, which has stood in the courtroom for much of the trial.
It was a partial re-enactment of the fatal night, with Pistorius claiming he felt fearful and vulnerable when he shot his 9 mm pistol at the toilet cubicle while only standing on his stumps. Then, describing the moment he fired the shots, Pistorius repeated that he was filled with terror because of the possibility of an intruder in the cubicle.
"I wasn't sure if someone was going to come out the toilet and attack me," he said. He testified he then heard a noise from inside the cubicle and said "before I knew it I had fired four shots at the door."
Pistorius spoke in a hushed voice, which sometimes quivered, and was asked on more than one occasion by the judge to speak up. He said after the shots he had searched for Steenkamp in his darkened bedroom, patting the bed where he claims he thought she was, searching on the floor next to it where he thought she might be hiding, and also behind the curtains.
"It was at that point ... that it first dawned on me that maybe it was Reeva in the toilet," Pistorius said, adding he then began screaming for help. "I don't think I've ever screamed like that ... I was crying out for the Lord to help me, I was crying out for Reeva."
Pistorius then sobbed loudly while testifying about how he eventually broke down the toilet door with a cricket bat to find a dying or maybe already dead Steenkamp inside.
Gerald Imray is on Twitter at www.twitter.com/GeraldImrayAP