sharp-horned fighting bulls is supposed to start at noon each year with a traditional shout of "Viva San Fermin!" and the launch of a firecracker known as the "chupinazo".
But just 10 minutes before the firework was to be set off from the city hall in a central square of the northern city of Pamplona, a massive Basque flag was hoisted in front of the building.
City officials struggled for 19 minutes to remove the flag, strung up between buildings on either side of the Plaza Consistorial square, before the firecracker could be set off.
"I am not going to tolerate setting off the chupinazo with a flag that is not the flag of Pamplona," said the city's mayor, Enrique Mayor, as tens of thousands of revellers, dressed in white and holding red scarves aloft, waited under a blazing sun.
"We have to do it the right way, and not with the indignity some want to impose on us," he told Spain's public television.
Pamplona lies in the Spanish Basque Country, where some favour creating an independent nation in northern Spain and southern France.
When the firework finally flew above the square, masses of people squeezed into the streets broke into cheers, danced and sprayed each other with sangria and cheap wine, turning white shirts pink.
Many onlookers peered from balconies overlooking the huge celebrations.
"It's one of those big things you need to get done before you die," said Alison Windsor, a 27-year-old Australian who came just for the festival.
"I needed to come once in my life," she said.
"I am not sure I will run with the bulls."
The festival, which dates back to medieval times, features religious ceremonies in honour of San Fermin, the patron saint of Pamplona, as well as concerts and round-the-clock drinking, with bars open until 6:00 am.
But the highlight is a bracing, daily test of courage against a pack of half-tonne fighting bulls thundering through the northern city's cobbled streets.
Each day at 8:00 am hundreds of people race with six huge bulls, charging along a winding, 848.6-metre (more than half-mile) course through the narrow streets from a holding pen to the city's bull ring, where the animals will be killed in a bullfight.
The first bull run, which traditionally draws the largest number of participants, is on Sunday. A run takes on average just under four minutes.
Last year 38 people were taken to hospital at 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, emergency services said.
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.
The most recent death took place four years ago when a bull gored a 27-year-old Spaniard in the neck, heart and lungs.
Hundreds of thousands of people flock to the city of 200,000 residents each year for the festival, made famous worldwide by Ernest Hemingway's 1926 novel "The Sun Also Rises".
The festival has not escaped the economic downturn gripping Spain. Pamplona city hall has slashed the budget for the fiesta this year by 13.8 percent to 2.1 million euros ($2.7 million).