The San Fermin festival starts on Wednesday in Pamplona with an 800m dash by hundreds of thrill-seekers in front of six bulls to the ring where the animals will be killed by matadors later in the day.

It is repeated each morning until 14 July in the Basque country city about 300 km (200 miles) north east of Madrid.

Gory images

But this year People for the Ethical Treatment of Animals (PETA) has launched a TV advertisement in the US featuring a Pamplona resident telling how his father took him to a bullfight as a child, mixed in with gory images of bulls being killed in the ring.

Hoof it: Trampling is commonplace

The group also plans to hang a protest banner saying "Stop the bloody bullfights" above the narrow streets through which the bulls charge on their way to the ring.

PETA spokeswoman Morgan Leyh called the festival a "tradition of tragedy".

Morgan Leyh: "It is a cruel sport"

She said: "Two weeks prior to going into the ring the bulls are debilitised with laxatives, they are beaten with heavy sandbags and kept in darkness so they are weakened by the time they go into the run.

"It is just a cruel blood sport. Tourists need to know that they are the same bulls that are going to suffer a slow brutal death in the bullring."

Ancient tradition

The festival, which dates back to 1591, has lured tourists by the thousands ever since Ernest Hemingway included it in his novel The Sun Also Rises.

And Pamplona's deputy mayor Jose Ignacio Laviano says the festival will continue.

"It is a very ancient tradition and the bull is a respected figure. It is not mistreated," he added.

Street fight: 13 runners have died this century

Local experts are joined by hundreds of less wary foreigners for the bull run.

The last fatality occurred in 1995 when a 22-year-old American was gored to death, but only 13 of the hundreds of thousands of runners have died at the festival in this century.

Minor goring and bruising from being caught and trampled under sharp hooves remains commonplace. Last year several foreigners were seriously injured, including a British man who was tossed on the horns of a bull.