A man is gored in his left leg in Pamplona on Sunday. (Alvaro Barrientos/Associated Press)

Four people were gored, two of them seriously, at the running of the bulls in Pamplona, Spain, on Sunday.

Bulls are let loose on the city's streets every day for the eight days of the San Fermin Festival and thrill-seekers can run alongside them.

During the sixth day of the festival, one of the bulls became separated from the pack, turned back and charged at runners.

One runner was gored in the neck. Another suffered a double goring in the upper chest and thigh.

The running of the bulls is an ancient tradition that was popularized by American novelist Ernest Hemingway. It has become Spain's best-known festival and many Spaniards consider it to be a defining tradition.

Animal rights groups want it banned, but enthusiasts say the thousands of runners who take part every day know what they're getting themselves into.

Fifteen people have been killed since officials began keeping records in the 1920s.

This year's festival began with hope that the running of the bulls was becoming less dangerous. The first few runs resulted in mainly bumps and bruises, but on Friday, a 27-year-old Spaniard died after he was gored.

A bull split from the pack in much the same way as happened on Sunday. The animal fatally gored veteran runner Daniel Jimeno Romero as he tried to flee underneath one of the wooden beams that fence in the route.