A straggling half-tonne bull gored two men as it turned its horns on panicked daredevils in a bull-run on the rain-slicked streets of Spain's northern city of Pamplona.
Another three runners were taken to hospital with bruises to the head and legs after 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 on Wednesday.
One 600kg bull named Brevito lagged behind the pack just before entering the city's bull ring at the end of the bull-run.
There, the huge animal confronted frightened runners, who were 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.
The bull gored a 32-year-old American from Chicago in the right thigh and a 35-year-old Spanish man in the chest before finally being guided away to finish the run.
Both men were in serious condition in hospital, officials said.
Regional health authorities identified the injured American only by his initials BH.
But Spanish media said he was Bill Hillman, a journalist and author who was taking part in the Pamplona bull-running festival for the 10th straight year.
Hillman published a book in June titled How to Survive the Bulls of Pamplona, which was co-authored with John Hemingway, the grandson of Ernest Hemingway whose 1926 novel The Sun Also Rises made the fiesta famous worldwide.
Emergency services workers erected red sheets around one of the men who was gored to protect his privacy as they applied first aid.
The wail of ambulance sirens filled the historic centre of Pamplona shortly after the run ended as the six injured were whisked to hospital.
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 adrenalin-charged - and often alcohol-fuelled - thrill seekers.
Dozens more runners were treated at the scene for scrapes and bruises suffered in falls along the winding, 849m course through narrow streets that were slippery due to overnight rain.
"I saw a lot of people fall along the way," said Juan Pedro Lecuona, a 41-year-old father of four who has run with the bulls in Pamplona every year since 1989.
"The bulls were very fast," added Lecuona, an auto sector worker who was gored in the leg at a Pamplona bull-run in 2010.
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 Spanish bricklayer in the groin. The man remains in hospital.
