Five foreigners are among six people gored in a hair-raising second running of the bulls at Pamplona's San Fermin festival.
A Navarra regional government statement said a Spaniard aged 58 and a 73-year-old South African man were in serious condition after being gored.
The regional government said an American, 55, and another aged 23 years were gored but their injuries were reported to be less serious.
An Indian aged 26 was also said to have sustained a less serious goring.
The regional government said a Canadian was gored in the bull ring just before the end of the run but it gave no details on his condition.
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It said eight others were also taken to city hospitals for other injuries suffered in the run.
Several of the six bulls used in the run got separated from the pack moments into the 8am run and began charging whatever came in sight.
One runner could be seen pulling a bull away by the horn to prevent him goring a woman and a man who had fallen to the ground. Another beast repeatedly tossed a man in the air before being lured away by other runners.
More than a thousand people took part in the run, which lasted nearly six minutes, more than twice the normal running time.
The bulls used on Friday weighed between 530 and 650kg.
The nine-day fiesta became world famous with Ernest Hemingway's 1926 novel The Sun Also Rises and attracts thousands of foreign tourists.
Runners dash along with six bulls down a narrow 850-metre course from a holding pen to Pamplona's bull ring. The bulls later face almost certain death in afternoon bullfights.
Bull runs, or "encierros" as they are called in Spanish, are a traditional part of summer festivals across Spain. Dozens of people are injured in the runs, mostly in falls.
Ten people were gored in the San Fermin festival last year.
In all, 15 people have died from gorings in the festival since record-keeping began in 1924.
