At least 21 people were injured during Spain's San Fermin bull run on Saturday when thrill-seekers fleeing the beasts were crushed at the narrow entrance to the bull ring, officials said.
As the huge animals thundered into the entrance of the tunnel, they were blocked by a mound of dozens of people who had fallen and were piled on top of one other.
One bull that had fallen before the entrance got up and charged into the clogged passageway. Two steers jumped over the pile of people as they began to get up and flee.
The blockage ended after attendants managed to let the beasts escape through a side door normally reserved for matadors.
Javier Sesma, a health spokesman for Navarra province, said two of the 21 injured people were gored by bulls and that the others were hurt in the stampede.
Mr Sesma said one runner, a 19-year-old Spaniard from Vitoria city, was seriously injured when his thorax was crushed at the bull ring entrance. An Irish citizen also suffered asphyxia. Mr Sesma said of the Spaniard: "It is a very grave situation. He's in a stable condition, but it's very serious."
One person was gored in his buttock and another in an armpit during the 850-metre dash through Pamplona's narrow streets, the official said. Neither injury was serious, said the Navarra government, which organises the annual festivities. The rest of the injured suffered cuts and bruises.
Mr Sesma said one spectator suffered a heart attack while watching the stampede.
On Friday, the festival drew widespread attention when an American college student and two Spaniards were gored, and videos and photos of the attacks were seen around the world.
The festival in this northern city dates back to the late 16th century and also is known for its all-night street parties. The runs, eight in all, are the highlight of a nine-day street festival to honor Pamplona's patron saint, San Fermin.