A university student has been killed and another seriously injured by stray bullets as gunmen shot dead two men in a parked car on a busy street near the centre of the South African city of Johannesburg, police say.

The students were hit when their bus was passing the scene of the shooting.

No arrests have so far been made.

Gun crime claims dozens of lives every day in South Africa, but witnesses to these killings in broad daylight said they were shocked by what happened.

"Both the men in the vehicle and one student were declared dead while the other student was taken to the nearest hospital for medical treatment," police spokesperson Col Dimakatso Nevhuhulwi told the BBC when describing what happened on Thursday afternoon.

A police statement said the students had been caught in "crossfire".

Images shared on social media show a man's body hanging out of a car window. Another shows what appears to be someone being treated at the scene on a main road in Braamfontein, a district close to a number of university campuses.

The University of Johannesburg said both students who were affected were studying at their institution. The undergraduate who was killed was 18 years old and travelling to his residence when he was shot, the university said. It has offered counselling to those who witnessed the shooting.

Wits University, which is located close to the scene, suggested the incident could have been related to taxi violence.

Police refused to comment on this, telling the BBC that investigations were ongoing.

A witness told local broadcaster eNCA she saw a gunman escaping in a white getaway car.

Johannesburg city police spokesperson Xolani Fihla said the motive for the shooting was not known.

A student told eNCA that she heard multiple gunshots, followed by chaos.

"It was a really traumatic experience. It is really scary because it could have happened to anyone," she said.

Another said she had witnessed previous incidents of violence in the area.

Police and forensic investigators are at the scene.

Between July and September last year, an average of 34 people were shot dead every day in South Africa, police crime statistics say.