The thrillseekers raced to keep ahead of six fighting bulls and six bell-tinkling steers tasked with trying to keep the beasts together along the 930-yard course from a holding pen to the bull ring in this northern city.
Several runners were knocked to the ground and some were trampled on by the animals weighing more than 1,100 pounds but there were no gorings or life-threatening injuries.
An 18-year-old man from Melbourne, Australia, suffered an eye injury and a 20-year old Spaniard suffered multiple contusions, Navarra state government said on its web site. Both were hospitalized but their injuries were not considered serious. They were identified only by their initials.
The 8 a.m. daily run is the highlight of the nine-day street drinking festival, and comes after a full day and night of hard partying.
Dozens of runners, dressed in the festival's traditional white shirts and pants with red sashes, sang a chant to a statue of San Fermin at the start of the route seconds before a firecracker rocket blast signaled the release of the bulls from the pen.
Waiting on a corner was retired American pilot Peter Rostow, who then dashed about 35 yards alongside the bulls on a cobblestoned street before taking cover in a doorway, his heart pumping with adrenaline. He drank only water the day before to prepare.
"I know bulls, but they came about a hundred times faster than I thought they would," said Rostow, 58, of Austin, Texas. "I wasn't prepared for that, and the intensity of the senses was overwhelming, the smell of the bulls, the sound of them running, and the fear."
The run, broadcast live on national television, lasted 2 minutes and 45 seconds, a relatively fast sprint that saw the bulls staying together and paying little attention to the runners.
Spaniard Alfonso Gamboa didn't run but said the race was considered a good one because the bulls stayed in a tight pack.
"They went quickly and together, and because of that the people could run well," said Gamboa, 50, a Pamplona businessman. "It was pretty, and there weren't a lot of drunks."
The bulls that ran Wednesday morning will be killed in the evening in the bull ring, and their meat served up in Pamplona's restaurants.
After the run, people packed the bull ring to chase and taunt young calves while others resumed partying around the town, swilling beer, sangria and wine on the streets and in jammed bars. The festival features eight bull runs in total.
The party's start was marred Tuesday when Basque separatists prohibited from displaying their flag on a huge stage unfurled a banner demanding that terrorists convicted of bombings and killings be moved to prisons closer to their relatives.
Police intervened, beating people with batons, and fistfights also broke out between supporters and opponents of the ETA Basque separatist group. Some hurled beer and champagne bottles at officers, but ended up hitting people in the crowd instead, witnesses said.
Police at the scene declined comment. The Diario de Navarra newspaper reported Wednesday that a 31-year-old Spanish woman suffered serious injuries after being hit in the head with a bottle.
But the party resumed quickly after the melee, masses of partiers drinking all night on the streets, yelling "San Fermin!" and "Ole!"
Dozens are injured each year in the morning runs. Most get hurt after falling, but some are gored and trampled by the beasts. Last year's festival saw the first fatal goring in nearly 15 years.