The moment Michael Brown was fatally shot has been described by the officer who gunned him down.

Darren Wilson has given his version of what unfolded on August 9th when Michael Brown was shot multiple times in a Ferguson street.

He alleges the unarmed, black 18-year-old initially assaulted him in his police car.

Wilson told ABC's George Stephanopoulos, Michael Brown was repeatedly punching and grabbing him through the window, so he pulled out his gun and warned the teen to get back.

"His response, immediately he grabbed the top of my gun. And when he grabbed it he said 'You're too much of a pussy to shoot me'.

"And while he's doing that I can feel his hand trying to come over my hand and get inside the trigger guard, and try and shoot me with my own gun.

"And that's when I pulled the trigger for the first time."

He says it was the first time he had ever used his gun.

It failed to go off.

After attempting to fire his weapon - believing he would not survive further punches from the teenager - Wilson says Michael Brown's aggression rose.

He says he was punched again, prompting him to again try and fire his weapon.

At that point, he says Michael Brown broke away and Wilson was able to get out of his car - using his radio to report what had happened.

The police officer then gave chase.

He has defended pursuing Michael Brown down the street, as the teen ran away from him.

"My job isn't to just sit and wait. I have to see where this guy goes.

He says it he felt it was his duty to give chase.

"That's what we were trained to do."

Wilson says Michael Brown then stopped and turned to face him.

"And as he does that his right hand immediately goes into his waistband and his left hand is a fist at his side. And he starts charging me."

Wilson denies eye witness reports that at this moment Michael Brown put his hands in the air.

He says the teenager ignored all commands to stop running, prompting him to fire a series of shots.

"I noticed at least one of them hit him. I don't know where, but I saw his body flinch a little.

"And after that I paused, and I again yell 'Stop! Get on the ground,' giving him the opportunity to stop.

"And he ignored all the commands and he just kept running."

Wilson says he fired another series of shots, again noticing Michael Brown flinch.

"He gets to about eight to 10 feet, and as he does that he kind of starts laying forward like he's going to tackle me.

"And I look down my barrel of my gun and I fired. And what I saw was his head, and that's where it went."

When asked by ABC's George Stephanopoulos whether he shot Michael Brown right in the top of his head, Wilson responded 'Yes'.

The police officer has described how he feared for his life before killing the unarmed teenager.

He says when he grabbed Michael Brown's forearm during the initial assault, he felt the immense power the 18-year-old had.

"The way I've described it is it was like a five year old holding onto Hulk Hogan. That's just how big this man was...He was very large. Very powerful man."

He says he paused as the teenager ran towards him, asking himself whether legally he could shoot him.

The conclusion he came to was yes - fearing he would be killed if he did not fire.

Wilson says he is sorry for Michael Brown's family.

But he says he has a clear conscience and there was nothing he could have done differently to prevent the killing taking place.

"The reason I have a clean conscience is because I know I did my job right."