Revellers running ahead of Victoriano del Rio Cortes fighting bulls on Estafeta Street at the San Fermin festival in Pamplona, Spain. Photo: Alvaro Barrientos/AP

An American who co-wrote a book called Fiesta: How to Survive the Bulls of Pamplona has become one of their victims after he was gored in a morning bull run at Pamplona’s San Fermin festival.

Bill Hillmann, a 32-year-old from Chicago and a long-time participant in the nine-day Pamplona street party, was gored twice in the right thigh during one of the daily bull runs, organisers said on their website.

The injury was serious but not life-threatening, the Navarra regional government said.

A 35-year-old Spaniard from Valencia was also in serious condition after being gored in the chest during the same run on the festival’s third day, the statement said.

Photographs showed Hillmann – dressed in the San Fermin event’s traditional white with a red neckerchief – being gored on the ground by a black bull as other runners scattered.

Tension spiked when the bull became separated from the pack in the final stretch.

British writer Alexander Fiske-Harrison, one of Hillmann’s co-authors and a fellow runner, said on his blog that the bull turned back and charged at runners.

Dozens of people are injured each year in the runs, most of them in falls. Fifteen people have died from gorings since record-keeping began in 1924.

The bulls are invariably killed in afternoon bullfights in the ring.

The street-partying festival was immortalised in Ernest Hemingway’s 1926 novel The Sun Also Rises and attracts thousands of foreign tourists.

The daily bull run starts at 8am and usually lasts around three to five minutes. It ends at the bull-ring, where the bulls are corralled before reappearing in the evening bullfight, when they are killed.

San Fermin has become a global tourist attraction, with tens of thousands of Spaniards and foreigners pouring into the Navarran capital. Many participants drink and dance all night.

Hemingway aficionado Hillmann travels to Pamplona every year to take part in the festival.

The black bull that gored him was the heaviest of the morning’s six bulls from the Victoriano del Rio ranch, weighing around 600 kilos.

A 27-year-old man from Madrid was the last person to be killed during the bull run after being gored in the neck in 2009. There have been 14 fatalities over the past century at the fiesta, which dates to the 13th century.