Pamplona, Spain - A straggling half-ton bull skewered two men on Wednesday as it turned its horns on panicked daredevils in a bull-run on the rain-moistened streets of Spain's northern city of Pamplona.

Another four runners were taken to hospital with bruises as they tripped over each other while racing ahead of six fighting bulls and six steers in the annual San Fermin festival, regional health authorities said.

One bull lagged behind the pack just before entering the city's bull ring and confronted frightened runners, dressed in traditional white clothes with red scarves around their necks.

At one point, the beast charged a man who had fallen and was cowering by a wooden fence. Another runner tried to coax the sharp-horned fighting bull away by pulling on its tail.

A 32-year-old American from Chicago was gored in the right thigh and a 35-year-old Spanish man was gored in the chest before the bull was finally guided away to finish the run.

They were both in serious condition in hospital, officials said.

A divided pack of bulls presents one of the greatest dangers in the bull runs that are the centrepiece of this centuries-old festival, leaving the huge animals disoriented and irritated by crowds composed of thousands of adrenaline-charged - and often alcohol-fuelled - thrill-seekers.

The fighting bulls from the Victoriano del Rio ranch took three minutes and 23 seconds to cover the 850m course from a holding pen to Pamplona bull ring, the longest time of the three daily bull runs held so far this year.

The bulls will face matadors and death in the afternoon in the bull ring.

In the first bull run of the festival, on Monday, a bull gored one 52-year-old Spaniard in the groin area. The man remains in hospital but his injury was not considered serious.

The San Fermin festival, a heady nine-day mix of partying and adrenaline-chasing, draws hundreds of thousands of people from around the world to Pamplona, a city of around 300 000.

Fifteen people have been killed in the bull runs since records began in 1911. The most recent death occurred five years ago when a bull gored a 27-year-old Spaniard in the neck, heart and lungs. - Sapa-AFP