The bulls run through Pamplona
Dressed uniformly in white tops and with red bandanas secured around their throats, a larger-than-average crowd - due to the run coinciding with the weekend - hurriedly navigated the cobbled streets of the Spanish city, with six massive bulls, as well as six hefty steers, in pursuit.
It took just four minutes and six seconds for the three-quarter tonne beasts to charge the 848.6-metre course, finishing in a holding pen adjacent to the city's bull ring - their fate, an eventual slaying in a bull fight.
Authorities confirmed there were no gorings during the opening run of Pamplona's San Fermin festival, but said four men had been treated for light injuries.
A 24-year-old Australian, a 44-year-old Briton, a 26-year-old American and a 36-year-old resident of Pamplona sustained injuries and were treated by medical authorities, according to race organisers.
Last year a total of 38 people were taken to hospital during the festival's eight bull runs, including four men who were gored by bulls.
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, with the most recent death taking place four years ago when a bull gored a 27-year-old Spaniard in the neck, heart and lungs.
For this years' runners, there was a heart-stopping moment when a lone tan-coloured bull hung back and stopped just before the bull ring as its path was blocked by people.
The bull turned around to face a dense crowd that had built up behind it, sending panicky runners scrambling over wooden fence barriers for safety.
Desperate to prevent the bull from charging into the crowd, herders with long sticks eventually enticed the bull into the ring.
The daily bull runs are the highlight of a nine-day mix of partying and thrill-seeking, which draws hundreds of thousands of people from around the world.
The festival in this city of 200,000 residents was made famous by Ernest Hemingway's 1926 novel The Sun Also Rises.
"Never again," said Jonathan Chadwick, 31, an engineer from Auckland, New Zealand, who said it was his "first and last time" to run. "It was really scary," he said.
Ade Erinkitola, a 47-year-old IT worker from San Francisco said it was his first time but he'd be back.
"It was really exciting, my heart was beating really fast. I didn't know what to expect," he said.
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.
The bulls that are run each morning are invariably killed in evening bull fights, and their meat is served up in Pamplona's restaurants.
The festival has not escaped the recession that has plunged Spain into a grave economic crisis. Pamplona city hall has slashed the budget for the fiesta this year by 13.8 per cent to 2.1 million euros ($3.46 million).