The week-long event kicked off with the midday firing of the traditional chupinazo firework rocket on the city hall balcony as revellers descended on the city in the northern Navarra region for an event dating back more than 700 years.
![Vladimir Putin has dominated Russian politics for a quarter of a century [Getty Images]](data:image/svg+xml;charset=utf8,%3Csvg%20xmlns%3D'http%3A%2F%2Fwww.w3.org%2F2000%2Fsvg'%20width%3D'100'%20height%3D'100'%20data-ring-placeholder%3D'1'%3E%3C%2Fsvg%3E "Vladimir Putin has dominated Russian politics for a quarter of a century [Getty Images]")
But the run-up to this year's event has been overshadowed by reminders of the gang rape of a woman during the 2016 edition. Sexual assaults also marred previous contests.
In the central Plaza del Castillo square, town hall publicity material bore the slogan "Pamplona, a city free of sexist assaults" and city officials have launched a mobile phone app for women to use if they believe they are in danger.
Municipal employees and members of feminist associations handed out red badges shaped like hands to participants, to raise awareness of sexual violence.
"We shall let nothing pass (in terms of unacceptable behaviour)," insisted Merche Labari, a 56-year-old housewife of 56 wearing one of the hand badges.
"It is for us, Pamplona, to say 'no, enough is enough,' through propaganda, fines, whatever it takes."
This year's campaign comes in response above all to the incident that sparked national outrage two years ago when five men calling themselves "The Pack" filmed themselves having sex with an 18-year-old woman.
Thousands of Spaniards protested in April after a court sentenced them for "sexual abuse" rather than the more serious offence of "sexual assault", a category that includes rape.
A decision to free the assailants on bail pending an appeal just prior to the start of the festival, meaning they could in theory return this year, was widely condemned.
At the 2013 San Fermin festival photos emerged of women festival-goers being groped by men.
To try to stamp out such problems this year a phone number is available 24/7 for people to report sexual violence and police will have CCTV footage to help them identify the perpetrators.
Since 2004, some 95 percent of complaints of sexual violence during the festival have resulted in the attacker being identified, according to a study by Navarra University.
"When a woman complains it is very probable we end up knowing who is behind what happens and that should encourage people to have great confidence in institutions" dealing with the issue, study co-author and sociologist Lohitzune Zuloaga told AFP TV.
Lingering fears over 'The Pack'
But the violence of the Seville-based "Pack" is still on many minds, not least after they obtained bail.
"There's no justice in their country," stormed Leire Delgado, 31, nursing a beer ahead of this year's festivities.
"If you let it happen, it's not rape -- if you don't they kill you. So what do you do, stop going out?" she asked rhetorically in allusion to the case of Nagore Laffage, a nurse killed ten years ago at the festival after refusing the advances of a man.
Maitane Salazar, a 27-year-old cashier, had the 2016 assault in her mind as she noted that "this year, I shall try never to be on my own, even going to the toilet."
Some lament that the assaults have tarnished the reputation of the whole city, taking the gloss off an event supposed to be a heap of fun washed down with large quantities of sangria.
'Lawless city'
"Elsewhere, they see us as a lawless city," said Delgado, dressed in white with a red neckerchief, the timeless uniform for all participants.
"There are rapes in Tenerife, in Seville ... it's just that elsewhere people say nothing. In Pamplona nothing goes hidden," she says.
"People tend to complain more than in other cities," said Zuloaga. "That doesn't necessarily mean there are more cases but that people are more sensitive."
Feminist collectives have tried to avoid overly focusing on the 2016 case.
"Zooming in on a sole case renders invisible other assaults," they suggested in a pre-festival statement.
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