Teenager Kadar Hussein was shot in the back of the head in a Penge car park after a chance meeting with a man he had an ongoing dispute with, a court has heard.
Trai Williamson - also known as Rumbles, aged 19, of no fixed address, is charged with his murder.
Mr Hussein, also 19, of Upper Elmers End Road, Beckenham, was shot in the head at 7.57pm on May 8 last year at the corner of Kenilworth Road and Westbury Road.
He was taken to King's College Hospital but died at 7.55am the following morning.
Williamson was arrested in Croydon and charged with murder on August 8.
On the day of the shooting, Mr Hussein met three friends in Snowdon Close, Penge.
At around 7.45pm, the four walked across Royston Field, towards the scene of the shooting.
Williamson was walking from the other direction with two other people.
Brendan Finucane, prosecuting, told the Old Bailey: "Kadar had taken a call from his mobile and had walked ahead to take the call.
"The defendant, who was walking just behind two other people, encountered Kadar in the car park.
"It was there that he shot him and that shot hit him in the head.
"Kadar was shot from a relatively short distance away, around 10 to 15 meters.
"The bullet entered the back left of his head and travelled through to his brain.
"His friends heard the shot being fired, and ran towards the car park.
"They saw the deceased lying in the car park in an awkward way.
"His eyes were partly open and blood was coming out of his mouth."
The court also heard one of the people Williamson was walking behind, 20-year-old Nicholas Murray, heard a voice which sounded like the defendant, saying “don't move Kadar” just before the shooting.
Mr Finucane said: "Mr Murray turned back to look and saw the defendant with his arms outstretched, holding what looked like a black sock in his hands.
"He saw the arms coming down to his sides, him spinning around and running across the park.
"He walked towards the car park and found Kadar lying in the middle of it.
"He tried to speak to him. There was a hole in the right hand side of his head.
"His head was swollen and blood was running from his right ear and nostril.
"An hour after the shooting he received an eight-second call from the defendant.
"The defendant asked: 'Is he alright?' He replied: 'No'. The defendant asked: 'Is he alright?' He replied: 'No. Stay away from me'."
Mr Finucane added: "The people who were there all heard the shot and then some of them saw the defendant running away.
"The defendant ran across Royston Field into Franklin Road and then disappeared.
"A gun was not found. The defendant took it with him as he ran away.
"There was a dispute, going back some months to the latter part of 2012 between the defendant and the deceased. The dispute involved drugs."
Williamson denies murder.
The trial continues.