Right at the start of the panicky, early morning bull run in Spain's famed San Fermin festival, one black bull ran ahead of the rest of the pack of six fighting bulls and six steers and charged five runners, skewering two of them in the legs.
The two male victims aged 36 and 46 were taken to hospital for treatment, organisers said. Neither were considered to be seriously hurt.
Emergency workers took another five people to hospital with trauma and bruising, none of whom were judged to be in a serious condition, organisers said.
The animals will face matadors and death in the afternoon in the Pamplona ring.
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"This bull run was so fast I hardly realised it," said David Rubio Blazquez, a 33-year-old factory worker from Villareal in eastern Spain who has been running with the Pamplona bulls for seven years.
As one black bull galloped through the crowds, it head-butted a young man in a blue shirt from behind, sending him flying to the ground and then trampling over him.
"The worst thing is other people. They push you. They grab you. They are almost more dangerous than the bulls," said 27-year-old insurance salesman Luis Meldero, who lives near Madrid.
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 Spanish man was gored.