Thousands of thrill-seekers dashed ahead of six fighting bulls in the streets of the northern Spanish city of Pamplona in a fast first running of the bulls in this year's San Fermin festival.

No one was gored, but four people were taken to Navarre Hospital with injuries - one with fractured ribs.

The injuries were suffered during a sprint where the six guiding steers stole the show from the charging bulls from the Torrestrella ranch, which is famed for producing dangerous bulls.

Runners, wearing traditional white clothing and red kerchiefs around their necks, tripped over each other or fell in the mad rush but avoided getting caught out as daredevils and animals charged down the near 850-metre course from a pen to the city's bull ring in two minutes, 30 seconds.

"Anything we heard before doesn't even compare to what really happened," said 28-year-old California native Allison Byrne, whose husband Brian ran for the first time.

"It was hard to watch that knowing that our husbands are down there. It was exciting and exhilarating, but I'll never do (this) again. I'm still shaking."

There were some unexpected moments as one of the steers separated from the pack and charged back toward the starting gate, sending runners scattering after having believed their dash was finished.

Another steer stopped and lay down on the pavement near the end before eventually being guided into the bullring, where the six bulls will take part in the afternoon's bullfight.

"The steers are like a herding dog to the bulls. They are there to organise and manage the course. If there were no steers, the bulls would stop charging after 100 metres. They are crucial to completing the run," said TV commentator Javier Solano, who has 37 years experience with San Fermin.

"What happened today is there were some young ones who don't know the course and a stray steer can be as dangerous as a bull," he said.

Runners filled the course for the 8 am start, with a thick crowd developing just after the halfway point, where one of the bulls became separated from the pack.

"The police did a very poor job of managing the course today, they left it all too late," said Solano, who believed fortune smiled on the runners. "Luck plays a very important part in every encierro (bull run) and today there was a lot of luck. Luck that that bull didn't charge at anyone."

People come from all over the world to test their bravery and enjoy the festival's non-stop street parties which lasts until July 14.