An American who co-wrote a book called Fiesta: How To Survive The Bulls Of Pamplona has been badly gored in the morning bull run at Pamplona's San Fermin festival.

A Spanish man was also gored in the Running of the Bulls, or encierro, where runners in red scarves and white outfits dash through the town's streets pursued by the huge animals.

The author, Chicago resident Bill Hillmann, 32, tripped and fell when a bull gored him in his right thigh, according to festival's website.

The black bull that gored him was the heaviest of Wednesday morning's six bulls from the Victoriano del Rio ranch weighing about 600 kilograms.

The Spaniard, a 35-year-old man from Valencia, was gored in the chest and taken to hospital in a semi-conscious state, officials said.

The two men are in a serious condition, the regional government said.

A further three men, all Spanish, were taken to hospital with lesser injuries from the chaotic stampede through the narrow streets of Pamplona's old town.

All the injured were men. Few women take part in the run.

Authorities did not give the names of the injured men.

Wednesday's run was the third in the week-long San Fermin festival which was famously depicted in Ernest Hemingway's 1926 novel The Sun Also Rises.

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

There have been 14 fatalities over the past century at the fiesta, which dates to the 13th century.

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.

Reuters