Olympic and Paralympic track star Oscar Pistorius reacts during judgement at the North Gauteng High Court in Pretoria, September 11, 2014.[Photo/Agencies]

PRETORIA - A South African judge cleared Oscar Pistorius of all murder charges on Thursday, saying prosecutors had failed to prove the Olympic and Paralympic track star intended to kill his girlfriend or an imagined intruder on Valentine's Day last year.

As judge Thokozile Masipa delivered her decision, Pistorius, who would have faced at least 25 years behind bars for premeditated murder, sat sobbing in the dock, tears streaming down his cheeks.

Pistorius lawyer: Athlete's anxiety led to killing

A verdict on the less serious charge of culpable homicide is still to come. Although Masipa described the 27-year-old as a "very poor" and "evasive" witness, she said it did not mean the track star was necessarily guilty 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 premeditated murder," Masipa told the Pretoria High Court. "There are just not enough facts to support such a finding."

She then proceeded to absolve Pistorius, who said he shot model and law graduate Reeva Steenkamp in the mistaken belief she was an intruder hiding in a toilet cubicle, of a lesser murder charge that falls short of a direct intention to kill.

"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.

"FICKLE HUMANS"

Although he has been cleared of the two murder charges, he could still be convicted of culpable homicide for the negligent or reckless killing of Steenkamp, hit by four 9mm rounds fired through the toilet door at Pistorius's luxury Pretoria home.

Culpable homicide carries up to 15 years in jail.

Alternatively the double-amputee could be acquitted, allowing him to leave the court and potentially resume his career as one of the biggest names in world athletics.

As the 66-year-old Masipa began her methodical review of the 41-day trial and the charges - which also include three unrelated firearms offences - a pained and forlorn Pistorius bowed his head in the dock.

Masipa, only the second black woman to rise to the bench in South Africa, has remained impassive throughout the often dramatic and gruesome court proceedings, seemingly impervious to the global interest in a case that has drawn comparisons to the 1995 murder trial of American football star OJ Simpson.

In one early blow to Pistorius, she said defence allegations of police contamination of the crime scene "paled into insignificance".

However, as she drew up a detailed timeline of the shooting, she questioned the reliability of multiple state witnesses, including that of a neighbour who testified to hearing the terrified screams of a woman before and during shots.

She also rejected a mass of instant messaging evidence presented by both prosecution and defence to suggest, respectively, that the couple's relationship was on the rocks or loving and strong.

"Normal relationships are dynamic and unpredictable most of the time, while human beings are fickle," she said. "None of the evidence of a loving relationship, or a relationship turned sour, can assist this court."