No one was gored, but four people were taken to Navarre Hospital with injuries - one with fractured ribs - sustained 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 849-meter (928-yard) course from a pen to the city's bull ring in 2 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 laid 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 organize and manage the course. If there were no steers, the bulls would stop charging after 100 meters (yards). They are crucial to completing the run," said Javier Solano, who has 37 years experience with San Fermin and comments on the "encierro" - the Spanish term for the run - for national television.
"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 a.m. (0600 GMT) start, with a thick crowd developing just after the halfway point, where one of the bulls became separated from the pack and was a standout figure amid a sea of white and red-clad runners charging down Estafeta Street.
"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 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 nonstop street parties which lasts until July 14. The festival was immortalized by author Ernest Hemingway in his 1926 novel "The Sun Also Rises."
"The truth is there were very few people for the first running, so it was quite comfortable," said 20-year-old Julen Iruzun, who was taking part in his third encierro.
"That must be one odd steer" Iruzun said about the wandering one, "but these things happen."