Thrill-seekers tumbled into each other in a scramble to escape six half-tonne fighting bulls thundering through the cobbled streets of Spain's northern city of Pamplona on Tuesday in a mad dash that injured at least two people.

The two injured men, Spaniards aged 30 and 47, suffered bruises in falls and were taken to hospital for treatment after the third bull run of the annual San Fermin festival, regional health officials said.

Runners jumped up and down to warm up and did stretching exercises before a firework set off the race, releasing six black bulls and six steers to charge through the city to the bull ring, where they were to be slain by matadors.

The bulls from the Valdefresno ranch took just two minutes and 29 seconds to cover the winding 848.6-metre (928-yard) course.

Hundreds of people, most clad in traditional whites with a red scarf around the neck, fled as the large, sharp-horned beasts charged along the route. Spectators watched from balconies and from behind wooden barriers.

"It all happened very fast. I tripped but I managed to land on my hands and did not hurt myself. I couldn't see anything, just people going by. It was different from what I was expecting," said Matt Barney, a 26-year-old bartender from Wilmington in the US state of New York whose trousers were dirtied at the knees from his fall.

A black bull pulled away from the pack and was the first to charge into the city's bull ring where it was quickly followed by the rest of the beasts.

One young man turned to face the breakaway bull and fell on his back, forcing other runners to scramble to avoid trampling him.

"I ran for the first time and I was really nervous but it was over so quickly that I almost did not notice. The most dangerous part was the people pushing you, I saw people fall because of that," said 18-year-old Luis Arana Ferrado who is from the nearby Basque city of Bilbao and was staying with a group of friends at a Pamplona campsite for the festival.

The daily bull runs are the highlight of a nine-day mix of partying and thrill-seeking that draws hundreds of thousands of people from around the world.

The festival in this city of 200,000 residents was made famous by Ernest Hemingway's 1926 novel "The Sun Also Rises".

The bull runs are believed to have started when butchers began running ahead of the beasts they were bringing from the countryside to the San Fermin festival.

Each year 200-300 people are injured in the bull runs, a few dozen requiring hospital care.

Most of the injuries are not caused by bull horns but by runners falling or getting knocked over or trampled by the animals.

Fifteen people have been killed in the bull runs since records started in 1911.

The most recent death took place four years ago when a bull gored a 27-year-old Spaniard in the neck, heart and lungs.