One man was left in a coma after being thwacked round the head by a giant rolling ball and another was hospitalized with three broken ribs after the annual festival in Mataelpino.
Video footage captured the moment of the worst injury, when a man was crushed between the ball and metal fencing lining the streets.
The town has been staging its ‘cruelty-free' alternative to the more common encierros for seven years using 200kg polistirene balls coated with a thick layer of resin and then painted with images of bulls.
But after the spate of injuries, the mayor of the town has promised a review of security measures.
“People who are used to bulls know that they can't just stand still in front of them because they could be gored. But with the ball some people think that nothing will happen to them, but this is not the case. If they get a direct hit, it can be serious,” the mayor, Javier Pérez de los Nietos, told El Pais.
“We are going to review all of our security measures and boost the information given out because people have started coming here from all over the world. We don't want this to become a tragic event, but rather something fun,” he said.
A message on the town hall website reported that the injured man remained in a critical but stable condition 72 hours after being knocked to the ground on Saturday.
Mataelpino, a small community of no more than 1,700 residents just outside the Spanish capital, was the first place in the country to hold a ‘running of the balls', an event that has caught on in other pueblos across Spain.
Dubbed boloencierro, a made-up up term which combines the Spanish words for bowling ball and running of the bulls, the event sees participants being ‘chased' down the sloping streets of Mataelpino by huge balls rolling at great speed.
The festival costs a fraction of the price to stage in comparison with the usual taurine events and Mataelpino has seen a rise in tourism numbers as a result.
It has also been championed by animal rights activists as a humane and just as fun alternative.