Video shown on Spanish media outlets showed the bull jumping several meters (yards) high out of the ring, clearing two barriers before landing in the stands and raising a panic as he lurched through the screaming crowd, charging and tossing everything he could.
The animal was brought under control after several minutes and later killed.
The incident occurred Wednesday at the Tafalla arena in the northern region of Navarra during a spectacle in which mostly young people try to get a bull to charge at them so they can dodge it with slick movements of their bodies. Unlike standard bullfights, the bulls are not killed in these events.
On Wednesday, the bull had already attempted to jump into the stands twice. After damaging a horn, he was about to be returned to the corral and replaced with another beast when he tried a third time and succeeded.
A regional government statement Thursday said three people remained in hospitals in the regional capital of Pamplona, best known for its annual San Fermin running of the bulls festival.
The injured included a 10-year-old boy who was in intensive care after the bull reportedly fell on him. Another man was gored in the back and was said to be in stable condition.
In all, 40 people were treated, primarily for minor injuries.
The regional government said the crowd was made up largely of young people, and most were able to get away from the bull quickly, avoiding a greater number of injuries.