"The accused asked for a firearm while in a restaurant full of patrons," Judge Thokozile Masipa said.
"He may not have intentionally pulled the trigger but that does not absolve him."
She added: "I view the state has proved beyond all reasonable doubt" that Pistorius contravened gun laws.
Pistorius was earlier cleared of another gun charge. The incident related to an allegation that the Olympian fired his weapon through a car sunroof in November 2012.
But the judge said prosecution witness Darren Fresco was "poor", and cleared Pistorius of the charge.
Legal experts said the state could question a ruling by the South African judge in the case of Oscar Pistorius, who was sensationally cleared yesterday of murdering his girlfriend Reeva Steenkamp.
LIVE STREAM: Pistorius verdict Day 2
As well as finding the Paralympian not guilty of premeditated murder, Judge Thokozile Masipa also absolved Pistorius, 27, of a lesser murder charge that requires a different concept of intention - known by the legal term "dolus eventualis" - which holds you responsible for the foreseeable consequences of your actions.
Earlier, the defence said the double amputee shot the 30-year-old model as a result of a tragic accident after mistaking her for an intruder hiding behind a locked toilet door.
"Clearly he did not subjectively foresee this as a possibility that he would kill the person behind the door - let alone the deceased - as he thought she was in the bedroom," said Masipa.
But Stephen Tuson, a law professor at Johannesburg's University of Witwatersrand, said the state would arguably be able to appeal on dolus eventualis (which is Latin for "indirect intent").
"How can you shoot four bullets through a door and not foresee their death?," he told Bloomberg News.
"Arguably, there's a possibility of an appeal by the state if they believe that she [Masipa] erred on a question of law."
However, the practising criminal barrister added: "I think the verdict on premeditated murder is acceptable and well reasoned and not a surprise."
Meanwhile, Professor James Grant, of the same university, said: "The problem is that Masipa has found that he did not intend to kill. But that is not the question and it was not his defence," he told The Citizen.
Judge Thokozile Masipa delivers her verdict. Photo / AP
"It seems as if she arrived at the conclusion by making a mistake of law relating to whether it matters who was behind the door."
Prof Grant added that it did not matter who was behind the door but that Pistorius shot at the door with intention.
While premeditated murder carries a minimum jail term of 25 years, dolus eventualis - also known as common murder - can lead to a maximum sentence of 20 years.
Pistorius now faces a troubled night after Masipa adjourned for the day before ruling on a charge of culpable homicide, which carries a maximum sentence of 15 years.
He had been accused of deliberately shooting his girlfriend during a furious row at his Pretoria home on Valentine's Day last year.
Oscar Pistorius and Reeva Steenkamp. Photo / Gallo Images / City Press / Lucky Nxumalo
But he had always maintained - often through wails of despair and vomiting - that he shot the law graduate in self-defence after mistaking her for an intruder.
Judge Masipa went on to address a lesser charge of culpable homicide - similar to manslaughter - saying the runner had been "negligent" when he fired the fatal shots.
She told the court that Pistorius had acted "hastily" and with "too much force" but in a moment of high drama abruptly adjourned proceedings for the day before reaching a verdict.
The Paralympian sat sobbing in the dock with tears streaming down his cheeks as the judge delivered her not guilty verdicts for murder and pre-meditated murder.
Delivering her verdict yesterday, Judge Masipa criticised Pistorius's decision to reach for his 9mm pistol and fire it through the toilet cubicle rather than raise the alarm or fire a warning shot.
She said: "There were other means available to you to deal with threats to life.
"All the accused had to do was pick up his cell phone and ring security or run to the balcony and call for help.
"Many people in this country have experienced crime or the effects thereof, directly or indirectly at some time or another.
Oscar Pistorius arrives at the high court yesterday. Photo / AP
"Many have been victims of violent crime but have not resorted to sleeping with firearms under their pillows.
"If the accused, for example, had awoken in the middle of the night and in darkness seen a silhouette by his bed and in a panic shot at that figure, only to find it was the deceased, his conduct would have been understandable and perhaps excusable.
"In such a situation he would not have been expected to call security first as he would be faced with a real emergency."
She added: "The accused had reasonable time to think, reflect and conduct himself.
"I'm not persuaded that a reasonable person with the same disability would have fired the four shots.
WATCH: Yesterday's judgment here
"The accused knew there was a person behind the toilet door, he chose to use a firearm.
"Would a reasonable person in the same circumstances as the accused have foreseen the possibility that if he fired four shots whoever was behind the toilet might be struck and die as a result?
"Would a reasonable person in the same circumstances as the accused have guarded against that possibility? The answer to both questions is yes.
"Did the accused fail to take steps which he should have reasonably taken to guard against the consequence? Again the answer is yes.
"I am of the view the accused acted too hastily and used too much force. It is clear his conduct is negligent."
Evasive witness
In coming to her decision on the murder charges, Judge Masipa earlier described the 27-year-old as a "very poor" and "evasive" witness.
But she said that did not mean the track star was necessarily guilty of the murder charges in a case she said was based entirely on circumstantial evidence.
"The state has not proved beyond reasonable doubt that the accused is guilty of pre-meditated murder," Masipa told the Pretoria High Court.
"There are just not enough facts to support such a finding."
She then proceeded to absolve Pistorius of the lesser charge of murder without pre-planning.
"Clearly he did not subjectively foresee this as a possibility that he would kill the person behind the door - let alone the deceased - as he thought she was in the bedroom," she told the packed courtroom.
Reeve Steenkamp. Photo / Ice Model Management
The judge said she believed accounts an emotional Pistorius gave to police at the scene in the moments after the shooting.
She said: "Counsel for the defence correctly argued that it was highly improbable that the accused would have made this up so quickly and be consistent in his version, even at the bail application, before he had access to the police docket and before he was privy to the evidence on behalf of the state at the bail application.
"The question is, did the accused foresee the possibility of the resultant death, yet persisted in his deed reckless of whether death ensured or not?
"In the circumstances of this case, the answer has to be no.
"How could the accused reasonably have foreseen that the shot he fired would kill the deceased?
"Clearly he did not subjectively foresee this as a possibility that he would kill the person behind the door, let alone the deceased - as he thought she was in the bedroom at the time."
Judge Masipa has yet to decide on a lesser charge of culpable homicide - similar to manslaughter.
Prosecutor Gerrie Nel had used them in an attempt to bolster his claim that the athlete shot his girlfriend in a fit of rage.
But the judge said that inference could not be made, adding: "Nothing of this proves anything at all."
She added: "Neither the evidence of the loving relationship or a relationship turned sour can assist this court to determine whether the accused had the requisite intention to kill the deceased."
Oscar Pistorius during the trial at the Pretoria High Court. Photo / Getty Images
Reeva Steenkamp's mother, June Steenkamp, listens to evidence. Photo / Getty Images
She also said there was 'some doubt' that a woman screamed on the night because of contradictory witness testimony, which she believed had been contaminated by media reports.
This apparently acknowledged the possibility that the defense argument that it was, in fact, Pistorius who had been screaming in a high-pitched voice.
However, she also cast doubt on the evidence of some defence witnesses.
And she said Pistorius himself in his evidence showed "a number of defences, or apparent defences".
At the start of the hearing, Pistorius appeared tense with a sick bucket by his side in the dock as Judge Masipa called Pretoria's High Court to order.
Early in her summary of the case, she dismissed a series of heated text messages between the couple which the prosecution claimed was evidence they were in a volatile relationship.
After reading a list of contradictory statements in Pistorius's testimony, she said one assertion "is inconsistent with someone who shot without thinking".
Pistorius wept in the dock as the judge described the injuries he inflicted on Miss Steenkamp when he shot her four times through a locked toilet door.
Media scrum
He arrived at the courthouse at 8am in a dark suit and white shirt, surrounded by bodyguards and police who escorted him through a scrum of reporters, supporters and television cameras.
He appeared calm, staring straight ahead and making no comment.
Shortly after, his estranged father Henke arrived, putting in only his second appearance at the trial that has captivated a global audience for the last six months.
"I'm just here to support him," Henke told reporters.
The athlete's brother, Carl Pistorius, arrived at the courthouse in a wheelchair with both his legs in splints in his first appearance since being seriously injured in a car crash earlier this year.
In a fitting reminder of how South Africa has changed in the 20 years since apartheid, the fate of Pistorius, a wealthy white man from privileged roots, rests in the hands of a 66-year-old black woman from Soweto.
Judge Masipa, only the second black woman to be appointed a high court judge in post-apartheid South Africa, analysed more than 4,000 pages of evidence before reaching her decision.
Oscar Pistorius celebrates as he wins gold in the Men's 400m at the London 2012 Paralympic Games. Photo / Getty Images
Oscar Pistorius during the medal ceremony for the Men's 400m at the London 2012 Paralympic Games. Photo / Getty Images
In arriving at her verdicts, Judge Thokozile Masipa had to weigh Pistorius's claim that he shot Ms Steenkamp after mistaking her for an intruder and the prosecution's allegation that the runner intentionally murdered her after a row.
After all the verdicts are delivered, there are likely to be arguments if and when sentence is handed down and, most likely, an appeal to a higher court.
Pistorius denied all the charges, including three other firearm-related counts.
Prosecutor Gerrie Nel took 15 days to lay out his case against the 27-year-old back in March, arguing he deliberately killed Miss Steenkamp by firing four rounds from a 9mm pistol through a closed toilet door.
The personality of the Paralympian gold medallist, who won worldwide fame when he competed on his prosthetic "blades" against able-bodied runners at the London Olympics, was a focus of the trial.
Prosecutors described him as an egotistical liar obsessed with guns, fast cars and beautiful women, who was not prepared to take responsibility for his actions.
Several neighbours testified to hearing a woman's terrified screams before a volley of shots, countering Pistorius's assertions that he mistook Miss Steenkamp for a burglar.
A case of two Oscars
Cutting through months of complex evidence and testimony, Mr Nel ended proceedings last month by returning to his core argument.
"He knew there was a human being in the toilet. That's his evidence," Mr Nel told the judge.
"His intention was to kill a human being. He's fired indiscriminately into that toilet. Then m'lady, he is guilty of murder. There must be consequences."
Defence lawyers, meanwhile, said there are "two Oscars" - a world-class athlete and a highly vulnerable individual with a serious disability who acted out of fear, not anger, when he fired the fatal shots.
Defence lawyer Barry Roux said during his own wrapping-up that psychological evidence had proven the track star had a heightened fight response because of his disability and was in a terrified and vulnerable state when he shot Steenkamp.
"You're standing at that door. You're vulnerable. You're anxious. You're trained as an athlete to react.
"Take all those factors into account," Roux said, adding that Pistorius had felt exposed because he was standing on the stumps of his legs.
"He stands with his finger on the trigger, ready to fire when ready.
"In some instances a person will fire reflexively," he added. "That is your primal instinct."
Mr Nel responded to claims that that the athlete suffered from an anxiety disorder by insisting that Pistorius undergo psychiatric tests to establish if he was mentally well enough to stand trial.
But, after a court-ordered 30-day assessment, experts ruled that he was capable of understanding the wrongfulness of his actions when he fired the fatal shots.
Mr Roux also argued that prosecutors had only called witnesses who supported their argument and not other key people, including police officers, who he said would have undermined their case.
The case also involved physical as well as oral evidence, with one forensic analyst demonstrating in court how Pistorius may have hit the toilet door with a cricket bat.
Another key element of the case was the time of Ms Steenkamp's final meal.
Pistorius said they ate at around 7pm on the night she was killed, and went to bed at around 10pm before the shooting in the early hours of the morning.
But the prosecution alleged that a finding that she still had food in her stomach after she was killed contradicted that story.
Both sides used texts sent by the couple to support their case.
In one read out by police captain Francois Moller, Ms Steenkamp said: \"I'm scared of u sometimes and how u snap at me and of how u will react to me."
But the defence claimed such messages were only a fraction of the total taken from the couple's mobile phones by police, and produced others in which they were affectionate.
For example, in January Steenkamp sent Pistorius a photo of herself in a hoodie with the message: "You like it?" He replied: "I love it."
"I don't hate Oscar"
The trial's tensest and most dramatic moments came in several days of highly charged testimony from Pistorius.
His voice thick with emotion, the athlete began his evidence by saying sorry to Ms Steenkamp's family.
Watched by the model's mother June, he said: "I was simply trying to protect Reeva. I can promise that when she went to bed that night she felt loved."
Mrs Steenkamp later told Hello! Magazine she has forgiven him. "I don't hate Oscar," she said.
Pistorius went on to describe how he had suffered from nightmares and sleeplessness following the incident, while also recounting the impact on him of previous instances of crime.
Later in his testimony, the court had to adjourn as Pistorius broke down sobbing and howling while describing the aftermath of the shooting. "I sat over Reeva and I cried," he said.
Pistorius was on the stand for five days of intense cross-examination from Mr Nel.
At one point he refused to look at a photograph of Ms Steenkamp's wounds as the prosecutor urged him to "take responsibility".
In July, the star's spokeswoman confirmed he got into a row about the trial while out in a nightclub, while in August his older brother Carl Pistorius was left in intensive care after a car crash.
In a tweet on August 8, Oscar Pistorius wrote: "Thank you to my loved ones and those that have been there for me, who have picked me up and helped me through everything."
- Daily Mail
