Celebrity athlete Oscar Pistorius arrived in court on Friday to face judgement for culpable homicide, a verdict that will decide if the famed "Blade Runner" walks free after his shock murder acquittal.

In a ruling followed live on TV screens worldwide, South African Judge Thokozile Masipa on Thursday declared the double amputee not guilty of deliberately shooting dead his 29-year-old lover, Reeva Steenkamp.

"The state clearly has not proved beyond reasonable doubt that the accused is guilty of premeditated murder," Masipa told the Pretoria High Court, before adjourning for the day.

On Friday she is expected to conclude her ruling and, if necessary, set a date for sentencing.

Flanked by bodyguards, the 27-year-old double amputee, who made fame and fortune from his sporting life before falling into scandal, pushed his way through the throng of media to get inside Pretoria's High Court.

If the homicide charge sticks, the Paralypian could face more than a decade in one of South Africa's notoriously violent jails.

Legal experts voiced surprise that Pistorius was found not guilty of murder, and predicted the case would likely not rest with the verdict.

Wits University criminal law professor James Grant noted the state could appeal if they believe there has been an legal error.

"Everyone is a little surprised," said lawyer Audrey Berndt, zeroing in Masipa's finding that Pistorius could not have known that firing four hollow point bullets through the bathroom door would have killed someone.

"She should have explained her reasoning a little more" said Berndt.

But Masipa -- whose career has taken her from a childhood in a poor Johannesburg township to the country's high court -- appeared to open the door for a lesser charge.

After describing Pistorius as a "very poor witness" who was "evasive" when questioned, Masipa criticised his actions in the early hours of Valentine's Day 2013.

"I am of the view that the accused acted too hastily and used excessive force. In the circumstances, it is clear that his conduct was negligent," she said.

"On the facts of this case I am not persuaded that a reasonable person with the accused's disabilities in the same circumstances would have fired four shots into that small toilet cubicle."

She also said that Pistorius "failed to take any steps to avoid the resultant death," crossing off another prerequisite for a manslaughter conviction.

Lawyers also expect that Pistorius will be convicted on separate charges of twice discharging a firearm in public and ammunition possession, for which he may also be jailed, lose his licence or face a fine.

- Relief and shock -

Following the first day of the verdict reading, the Paralympian sprinter hunched over in the dock and buried his head in his hands.

His sister Aimee rushed from the first row of the public gallery to hug her older brother, who was wiping tears from his eyes with a handkerchief.

His aunt Lois cautioned that the trial was not yet over. "It's not the end you know, we're still listening."

When asked how proceedings went, defence lawyer Barry Roux smiled, but it was short of his usual toothy grin.

The parents of Reeva Steenkamp, a law graduate voted one of the world's sexiest women by FHM magazine, left the courtroom with stony faces.

The trial, which has gripped South Africans and much of the world for half a year, has cast a harsh spotlight on the fallen hero's private life.

Full of high drama, the trial has fed intense media interest worldwide, with live broadcasts veering into the realm of TV reality.

During proceedings Pistorius has broken down, weeping and at times vomiting as he heard how his girlfriend's head "exploded" like a watermelon under the impact of his hollow-point bullets.