A 20-year-old American was forced to undergo a splenectomy after being gored here Friday during the sixth running of the bulls at Spain's San Fermin festival.

The man, identified only as Patrick E., was the most seriously injured out of three people gored during the slow and extra-dangerous run, suffering internal bleeding when a bull's horn pierced his abdomen with a upward blow, according to health authorities in the northern Spanish region of Navarre.

The man's spleen had to be removed due to the seriousness of the wound, but his condition is now listed as stable and non-life-threatening.

Two Spanish men also were gored: one aged 42 who was struck by a horn in the right arm and another aged 31 who was pierced three times but is listed in stable condition.

An American man and three Spanish men also received medical treatment for various injuries during Friday's run, which featured bulls from the Pilar ranch in the northwestern city of Salamanca.

Friday's gorings were the first of this year's San Fermin, ending a record streak of five days without any runners being pierced by horns.

The run began quickly and orderly but took a dangerous turn when one of the animals became separated from the herd and charged at several runners. Clocking in at four minutes and 57 seconds, the run was the longest of this year's festival.

The San Fermin festival, which started on Saturday and ends on July 14, when the last of eight running of the bulls will take place, is known around the world for the bull runs and uninhibited street revelry.

Pamplona, a normally quiet town, welcomes hundreds of thousands of Spanish and international visitors annually during the festival, which began about 400 years ago and was popularized by Ernest Hemingway in his 1926 novel "The Sun Also Rises."

Besides the bull runs, hundreds of events have been scheduled for this year's edition of San Fermin.

The run through the medieval streets of Pamplona's historic center, usually lasting four minutes, is especially dangerous because some runners take part in the event after all-night drinking binges.

This makes runners reckless and more likely to get too close to the bulls, which weigh in excess of 500 kilos (1,100 pounds).

The running of the bulls is monitored by experts who control the route and try to prevent accidents, but, inevitably, runners fall, suffer cuts and bruises, and are even gored by the animals.

Four people were gored at the 2012 San Fermin festival, while 41 were transported to hospitals and 388 others were treated at the scene by paramedics for a variety of injuries. EFE