The nine-day mix of alcohol-soaked partying and fleeing huge, 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.
"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.
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Pamplona lies in the Spanish Basque Country, where some favour creating an independent nation in northern Spain and southern France.
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.