Mr Beshira, 21, was pronounced dead on Kilburn High Road in north-west London at 17:20 GMT on Thursday after being shot in the abdomen.
A 22-year-old man was arrested on Saturday on suspicion of murder and remains in custody.
Metropolitan Police said they believe Mr Beshira was shot on Palmerston Road and staggered 200 yards.
Officers are searching for a man described as slim and white who was wearing jeans and a dark top with his hood up and the lower half of his face covered.
Members of the public were already attending to Mr Beshira, who had collapsed outside a cafe, when the emergency services arrived.
Witnesses described the area as "mayhem" after the shooting with reports of fights breaking out in a crowd.
Det Ch Insp Andy Partridge said the gunman was "loitering in and around Palmerston Road and the Webheath Estate prior to the attack".
He said: "We know the area was extremely busy at the time of the shooting.
"I am convinced that other members of the public have either seen the incident itself, or have seen the suspect arrive at the scene or flee after the shooting."
Detectives have identified a man who was shot and killed on a busy street as Yasir Beshira.