The Minneapolis police officer who shot dead Australian life coach Justine Damond-Ruszczyk in a dark alley feared he was the victim of an ambush, a jury in the US has been told.

Mohamed Noor, 33, has been charged with second-degree murder, third-degree murder and manslaughter in the 2017 shooting death of Ms Damond in Minneapolis.

Noor’s lawyer, Peter Wold, told the jury in the defence’s opening address on Tuesday that Ms Damond died in what “was a perfect storm with tragic circumstances”.

“It was a classic ambush scenario set-up,” Mr Wold, describing the scene of the shooting, told the jury that will decide Noor’s fate.

Ms Damond called 911 just before midnight on July 15, 2017 when she heard a woman’s screams near an alley behind her Minneapolis home.

The court heard how Noor and his partner, Officer Matthew Harrity, pulled into the alley, turned off their car’s lights, rolled down their windows and drove along the dark, narrow corridor.

Mr Wold said when they reached the end they did not hear or see anyone in distress.

The officers were also not aware of the identity of the 911 caller, adding to the possibility it was an ambush.

Mr Wold then described how Ms Damond suddenly appeared in the darkness and thumped the police car and a terrified Officer Harrity said “Oh Jesus” and grabbed his gun.

“Noor shoots once to protect his partner and himself,” Mr Wold said.

“It was a perfect storm with tragic circumstances.”

Justine Damond was dressed only in her pyjamas when she was fatally shot. Photo: Instagram

Noor, sitting in the front passenger seat, shot across Officer Harrity and fatally struck Ms Damond in the stomach.

Mr Wold told the jury Ms Damond’s Australian background added to the perfect storm.

“She came from Australia where gun violence was virtually non-existent,” Mr Wold said.

Noor was fired from the police force last year when he was charged.

Prosecutor Patrick Lofton, in his opening address, told how Noor and Officer Harrity only turned on their body cameras after the shooting.

He also said when other officers came to the scene, their body cameras were switched off and on.

“When (the) body cameras go on, there is a lot of silence,” Mr Lofton said.

Initially, there was no mention of Ms Damond slapping the police squad car, the prosecutor said.

He also said there was no forensic evidence Ms Damond touched the car.

It was only days later in an interview with investigators that Officer Harrity mentioned Ms Damond hit the car, the prosecutor said.

The trial is expected to last three weeks.

Ms Damond’s father, John Heffernan and stepmother Maryan, sat in the front row of the courtroom.