PAMPLONA, Spain (AFP) - Half-tonne fighting bulls knocked over thrill-seekers on Monday in a fast, adrenaline-fuelled chase through the cobbled streets of Pamplona that left four injured in Spain's San Fermin festival.

Thousands of people fled as the sharp-horned beasts charged along the winding 848.6m course in a quick two minutes and 25 seconds.

"We did not realise how big they would be. They were massive. They once turned round and came back again. Oh my God, it was crazy, indescribable," said 25-year-old Scot Ellis of London, who was with a group of friends.

"We are on vacation. Stupidest vacation we've ever done," he added. Four people were taken to hospital - a 41-year-old American from Phoenix and three Spaniards aged 35, 37 and 48 - but none had been skewered by the bulls' horns, regional health authorities said.

Two were discharged after a few hours but the American remained in hospital in serious condition with a fractured right shin bone along with the 35-year-old Spaniard who suffered a head injury in a fall.

It was the second bull run of the San Fermin festival, a heady nine-day mix of partying and derring-do that draws hundreds of thousands from around the world.

As tens of thousands of spectators looked on, many watching from overhanging balconies, runners dressed in traditional white with red kerchiefs fled the charging animals.

Some people sought safety by crowding close to the walls of the winding route but others dared to touch the rear of the bulls or to race a few steps in front of the sharp horns.

About a minute into the run, when the bulls took a sharp right, they swerved close to a wall on the left.

One young man slipped and fell just as the bulls approached. Then the bulls toppled a group of three or four others, leaping over some of the fallen runners.

"That was brilliant but scary," said Andrew Scoates, 25, from London.

"It was a very fast run," said Joaquin Subasti, a 52-year-old resident of Pamplona who has taken part in 39 of the bull runs.

"The bull run is always dangerous. There are gorings, broken bones, falls. It is a high-risk activity," Subasti said.

The festival in this northern 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.

Last year, 20,700 runners took part in the eight bull runs, an average of 2,587 a day, and some 38 people were taken to hospital including four men who were gored.

Several hundred more were treated for minor injuries at the scene.

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.

On Monday a 26-year-old Spanish man from the northeastern city of Zaragoza who came to Pamplona for the festival plunged two to three metres from a medieval wall into a river and fractured his vertebra, the regional government of Navarra said in a statement. He was in serious but stable condition in hospital, it added.

Squeezed by the Spanish recession, Pamplona city hall has slashed the budget for the festival this year by 13.8 per cent to 2.1 million euros (S$3.4 million).