Fighting bulls gored six people in a hair-raising second running of the bulls at Pamplona's San Fermin festival on Friday, the Spanish Red Cross said.

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, creating many moments of fear and tension.

Television images showed a runner pulling one bull away by the horn to prevent him goring a woman and a man who had fallen to the ground. Another animal was seen repeatedly tossing a man in the air before being lured away by other runners.

More than 1,000 people took part in the run, which lasted nearly six minutes - more than twice the normal running time.

The bulls involved on Friday weighed between 530kg and 650kg.

The Red Cross, which tends the injured in the immediate aftermath of the runs, said six participants were gored, updating initial hospital reports of five gorings.

In a note on its official Twitter account, the Red Cross said eight other people were also taken to city hospitals for treatment to other injuries sustained in the run.

There were no immediate further details.

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 850m 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, including four Americans, 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.