Some runners were gored and others injured during falls as they were chased down by the half-ton bulls in the streets of Pamplona, Daily Mail Online reports.
A 30-year-old Brit was rushed to hospital after being stabbed in the pelvic area. He was identified only by the initials A.B.O. His injuries are not thought to be serious.
One man narrowly escaped serious injury after one of the bulls grazed his chest with its horn near the famous Mercaderes corner on the 849-metre course.
Five Americans aged 21 to 28 and five Spaniards were also among the casualties in the first of the eight morning runs through the streets of Pamplona.
One of the Americans, a 38-year-old man, was gored in the armpit. He is thought to have been hurt after a black bull grazed his chest with its horn near the famous Mercaderes corner on the 849-metre course.
Red Cross workers were pictured taking several bloodied runners away on stretchers.
A spokesman for the hospital complex said: "We have one person who has been gored in the chest, who is stable and will be operated on. The others have suffered facial traumas or injuries to arms and legs."
The Pamplona bull run is the most popular in Spain and has been broadcast live by RTVE, the public Spanish national television channel, for over 30 years. It is the highest profile event of the San Fermin festival, which is held every year from July 6 to July 14.