A red-and-white sea of revellers soaked each other with wine in a packed Pamplona square on Friday, kicking off Spain's most famous bull-running fiesta.
A shout from the city hall balcony of "Viva San Fermin!" and the lighting of a firecracker known as the 'chupinazo' at noon launched the bedlam, which marks the official start of the nine-day street party.
"This is madnesss, total and complete madness. I love it!" said Louise Haley, a 28-year-old who came from Australia for the San Fermin festival and whose hair and clothes were drenched with wine.
Many thousands of revellers from around the world carpeted the Plaza Consistorial, their traditional white outfits, and red neck scarves, splashed with the stains of wine and sangria.
The crowd sang and danced and passed vast inflatable balls over their heads as scores looked on from crowded apartment balconies.
One young man pulled his pants down, provoking cheers and a flurry of photo-snapping on mobile phones.
Many party-goers got an early start to the drinking, swigging back sangria and beer in the morning ahead of the San Fermin festival, popularised worldwide by Ernest Hemingway's 1926 novel The Sun Also Rises.
They sat in outdoor patios or wandered the narrow, cobbled streets of the city carrying plastic cups - or sometimes even buckets - of alcohol as they sang.
"This is what breakfast should be like," said John Moorse, a 26-year-old tourist from California as he sat at a table with friends, a large plastic cup of beer and a pack of doughnuts in front of him.
The festival, which dates back to medieval times, features religious processions, folk dancing, concerts and round-the-clock drinking, with bars allowed to stay open until 6am.
But the highlight is a bracing, daily test of courage against a thundering pack of half-tonne, sharp-horned bulls.
Each day at 8am hundreds of people, most wearing white clothing and red kerchiefs around their necks, race with six fighting bulls along a 850-metre course from a holding pen to Pamplona's bull ring.
The bravest - or most foolhardy - run as close as possible to the tips of the horns, preferably without being gored.
The first bull run, which traditionally draws the largest number of participants, was on Saturday.
Ander Goyoaga, a 34-year-old unemployed plumber from Bilbao who has taken part in six bull runs in the past, said he planned to repeat the experience this year.
"It is an incredible sensation, your heart starts beating faster. Each time I say it will be the last but then I can't resist doing it again," 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.
Dozens of participants are injured each year. Most of the injuries are not caused by bull horns but by runners falling, getting knocked over or trampled by the animals.
The most recent death took place three years ago when a bull gored a 27-year-old Spaniard in the neck, heart and lungs.
The city of 200,000 residents expects the festival will draw half a million visitors and bring in more than €70 million (NZ$108m) in tourism earnings.
"I have seen the images on TV and I had to come. This has to be the world's wildest party," said Britain's David Higgins, 25, who came with three friends.
Last year 20,500 people took part in the eight bull runs of the festival, the majority of them men between the ages of 18 and 35, according to Pamplona city hall.
Nearly half of all participants came from abroad, with the United States, Australia and Britain accounting for the greatest number of foreigners.