At least 23 people were crushed after being trapped at the narrow entrance to the bullring by the bulls that had chased them through the streets for Spain's Pamplona festival, including an Australian woman.
The 23-year-old woman was gored in the chest on the final day of Spain's San Fermin week-long bull-running festival.
The woman was taken to hospital for surgery and was in a serious condition, according to reports. Three men were also gored during Friday's run, including an American tourist who had to have his spleen removed. One man was gored in the armpit and another in the buttocks, medical officials confirmed.
Local authorities said a 19-year-old Spaniard was being treated in hospital and was in a "very serious" condition. A runner from Ireland also suffered trauma to his chest, although his injuries were not thought to be as serious.
The stampede is believed to have occurred when some of those taking part in the street run from the bulls tripped at the front before the bulls then ploughed into the back of the scrum. Chaos ensued as runners and bulls became trapped side by side, leaving hundreds trying to scramble away.
The bull run at San Fermin - made famous by Ernest Hemingway in his novel "The Sun Also Rises" - is one of hundreds in Spain every year and attracts large numbers of foreign visitors.
The Pamplona run takes place at 8am every morning for one week in July until Sunday. Saturday's run lasted 4 minutes and 15 seconds.
Goring happens from time to time but stampedes are rare and only 14 have died in the last 100 years in the San Fermin run - a festival that that dates from the 13th century.
Runners dress all in white with red neckerchiefs and many spectators stay up drinking all night in bars beforehand.
After each morning bull run, the animals are usually killed by bullfighters in the ring.
Additional reporting by agencies