Bulls and humans alike slipped and slid on the cobblestone streets, but the first of Pamplona (search)'s annual runs with the bulls was largely injury free Monday as thousands of daredevils sprinted with the half-ton animals.

Some runners, sounding disappointed, said they'd seek out greater danger in coming days as the wildly popular, centuries-old San Fermin festival (search) serves up six more runs this week.

"It was a bit timid, to be honest. Nowhere near as scary as in the books," said Johnny McFarlane, 26, from Temuka, New Zealand (search), here for the first time after reading about San Fermin on a Web site.

"Tomorrow we're going to try and run a bit closer to the bulls and get a bit more adrenaline," added his friend, Richard Innes, 26, cradling the day's first beer.

The streets were slightly slick with morning dew as six fighting bulls and six steers dashed from a corral along the 900-yard course through the city's old quarter to the bull ring.

No one was gored, although four people who were trampled were hospitalized for treatment of head, chest, rib or leg injuries, the Navarra regional government said. None was seriously hurt.

The steers are meant to keep the bulls more or less in a cluster -- a spooked, isolated bull is very dangerous -- and that's pretty much what happened Monday.

At two sharp turns, several bulls slipped and went down with a heavy thud, and two bulls were separated from the pack. And a minor pileup of fallen runners formed at the tunnel leading into the bull ring. Bulls jumped or stepped over the runners.

But along the long, final stretch to the bull ring, some of the bulls and steers trotted in three perfect, tight-knit rows.

Veteran local runner David Zazpe, 29, said clean runs lead to overconfidence.

"That's the problem. People have one good run, and then they start taking risks," said Zazpe, who's been running with bulls for 13 years.

Since record-keeping began in 1924, 13 people have been killed at the San Fermin festival. The last fatality was 22-year-old American, gored to death in 1995.

Thousands of people watching in the street and from balconies shouted as the bulls rumbled through the city's old quarter, taking just over two and a half minutes to cover course.

Chase Hutto, a senior policy adviser for U.S. Energy Secretary Spencer Abraham in Washington, D.C., said running with the bulls was a little scarier than working in politics. "Congressmen generally don't charge, and their horns aren't as sharp," he said. "This is a rush you can't find anywhere else."

The fiesta, famed for its all-night street parties, dates back to the late 16th century but gained world fame from Ernest Hemingway's 1926 novel The Sun Also Rises.

Running bulls through the town began as simply the easiest way to get them to the ring for bullfights, but eventually daredevils started running in front of them.

Tens of thousands from all over the world have been pouring into Pamplona for the annual festival ever since, with an estimated 1.5 million attending over the course of the festival.