Police say the citizen was helping a 54-year-old man who had just been punched. At the same time the officer arrived at the scene, police received a call reporting a fight at the same location.
According to the witness, the victim was punched by another man while the two were talking. The victim fell to the ground and the suspect ran off.
The victim was rushed to the hospital and admitted into surgery for head injuries.
Several hours later, units checking the area near where the fight occurred located and arrested the suspect, identified as 42-year-old Derek Allen, of Ventura.
Anyone who may have witnessed the fight or who can provide more information on what happened is urged to call Ventura Police at (805) 339-4416 or Crime Stoppers at (800) 222-TIPS.