Two Americans and a Spaniard have been gored during the first running of the bulls of this year's San Fermin Festival, medical officials in the northern Spanish city of Pamplona said.
A 46-year-old Spanish man was undergoing surgery for serious injuries to his head and leg after he was gored and then tossed by a bull.
Hospital officials said two men from the United States had also been gored during the run but their injuries - in the abdomen for a 35-year-old man and in the scrotum for a 29-year-old - were not considered serious.
More runners were treated on the spot for bruises, a Red Cross spokesman said, as two of the animals separated from the other four in the pack on their way to Pamplona's bullring.
They completed the 930-yard (850m) course through cobbled streets in just under three minutes, a speed that is considered average for the run.
Friday's bulls came from the Cebada Gago ranch, famed as fierce because they have traditionally caused more injuries. Since they debuted in Pamplona's narrow streets in 1985, bulls from the southern Spanish breeding family have gored 53 people.
The nine-day San Fermin fiesta was immortalised by Nobel Literature laureate Ernest Hemingway in his 1926 novel The Sun Also Rises.