Three men were gored by half-tonne bulls as they raced through Spain's northern city of Pamplona on Friday, the bloodiest run of this years's celebrated San Fermin festival.

The three runners, a 20-year-old American and two Spaniards aged 31 and 42, were raced to hospital, regional health officials said.

Another three people, including a 48-year-old American, were also taken to hospital with injuries suffered in falls during the sixth bull run of the hugely popular festival.

The bull run turned dangerous when a half-tonne black bull, part of the galloping herd of six sharp-horned bulls and six steers, separated from the pack and began charging into runners.

The lone bull charged the 31-year-old Spaniard and tossed him around the ground in a 30-second tussle, causing the daredevil's pants to come off.

The man, from the town of Morella in eastern Spain desperately clung to one of the bull's horns, while two runners tried to pull the beast away by the tail.

Two others then grabbed the man by the feet and pulled him away to safety.

The runner was gored three times receiving injuries to the leg and groin.

An isolated bull is more dangerous because it can become disoriented and is more likely to charge.

The bull then knocked another runner near the entrance to the bull ring, sending him reeling against the wooden fence that lines the route.

"A lot of people fell along the route, it was really dangerous," said Alan Carpenter, a 38-year-old banker from Chicago after taking part in his third Pamplona bull run.

Hundreds of runners, most dressed in white with red kerchiefs, joined in the 848.6-meter (928-yard) dash through city's winding, cobbled streets from a corral to the bull ring.

Some dared to run just an arm's length in front of the bulls, glancing nervously behind at their dangerous curved horns while others chased behind the pack in the run watched live by millions on Spanish TV.

The six bulls from the El Pilar ranch in the province of Salamanca completed the course in four minutes and 57 seconds, the slowest bull-fun so far this year.

The animals, which weigh between 510 and 575 kilos, will be killed by matadors in an afternoon bullfight.

Sixteen first aid posts are set up along the route -- roughly one every 53 metres -- each with its own ambulance to whisk the more seriously injured to hospital if needed.

In total 21 men have been sent to hospital during the six bull runs held so far this year.

Dozens of other participants have been treated at the scene for scrapes and bruises.

The early morning bull runs are the highlight of the San Fermin festival, which was immortalized in Ernest Hemingway's 1926 novel "The Sun Also Rises" and now draws hundreds of thousands of tourists.

Fifteen people have been killed in the bull runs since records started in 1911.

The most recent death took place four years ago when a bull gored a 27-year-old Spaniard in the neck, heart and lungs.