* Pistorius enters Pretoria court
* Track star expected to plead 'not guilty'
* Trial likely to run for weeks (Adds details, background)
PRETORIA, March 3 (Reuters) - "Blade Runner" Oscar Pistorius arrived at the Pretoria High Court on Monday for the start of his murder trial, opening a decisive chapter in the story of the rise and fall of one of the world's best-known athletes.
The South African Olympic and Paralympic track star shot and killed his girlfriend, model Reeva Steenkamp, on Valentine's Day 2013. The 27-year-old argues that it was a tragic case of mistaken identity and that he mistook Steenkamp for an intruder hiding in the toilet.
Barring a last-minute change of heart, Pistorius, who appeared in court in a dark suit with a white shirt and tie, will enter a plea of "not guilty".
Prosecutors will seek to prove that Pistorius - known as "Blade Runner" after his carbon-fibre running prosthetics - fired four rounds from a 9 mm pistol through the door of the toilet adjoining the bedroom of his luxury Pretoria home in a deliberate attempt to kill whoever was behind it.
Steenkamp, a 29-year-old law graduate, women's rights campaigner and regular on South Africa's celebrity party scene, was hit three times, in the head, arm and hip. She was declared dead on the scene.
If the state succeeds in convincing Judge Thokozile Masipa of intent to kill, Pistorius will go to prison for life, in all likelihood a minimum of 25 years behind bars.
Besides the murder charge, Pistorius has already admitted to culpable homicide, equivalent to manslaughter, a crime that could see him put away for 15 years - or he could leave the Pretoria High Court a free man, with no more than a slap on the wrist and a suspended sentence.
Coming less than a month after the rape, disembowelling and murder of a teenager near Cape Town, the 2013 shooting of Steenkamp caused outrage and drew further attention to the high levels of violence against women in South Africa.
The trial before Masipa - juries were abolished by the apartheid government in the 1960s - is set to last a minimum of three weeks but with as many as 107 witnesses waiting to be called by either side it is almost certain to last far longer. (Reporting by David Dolan; Writing by Ed Cropley; Editing by Pascal Fletcher)