The incident occurred on 28 March when Mark Lang, the delivery driver, was dragged along North Road, Cardiff, and died weeks later.
Christopher El Gifari, 31, has pleaded guilty to manslaughter and theft but denies charges of murder and robbery. Seren Williams, who was on her lunch break at the time, testified that she saw a white van going over an obstruction and noticed something "luminous" under the van.
When she asked a man nearby what was happening, he ran off quickly. She and a colleague went to the van and called 999 after discovering that a person was trapped underneath. They felt helpless and did not attempt to move the van to avoid causing further injury.
The court also heard that Mr. Lang had left his keys in the ignition while delivering parcels in Laytonia Avenue. CCTV footage showed Christopher El Gifari getting into the van and driving off at high speed, with Mr. Lang chasing him.
Mr. Lang, wearing a high visibility jacket, was seen standing in the road, gesturing for the van to stop. The prosecution alleges that the defendant ran him down without deviation.
The van continued for almost half a mile with Mr. Lang trapped underneath before stopping near Cathays High School. Christopher El Gifari exited the van and fled the scene.
Mr. Lang suffered severe injuries, including head injuries and skin damage from being dragged along the road. He died 18 days later due to a brain injury, traumatic cardiac arrest, and blunt force injuries to his trunk.
Prosecutor David Elias KC described the incident as the "cowardly killing" of Mark Lang, who was trying to protect his and others' property. The trial is set to continue on Monday.