balcony of city hall of "Viva San Fermin!" and the launch of a rocket known as the "chupinazo".
Under a hot sun, partygoers and daredevils from Spain and around the world -- most dressed in white with red scarves -- jammed the streets, dancing and spraying 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.
Revelers celebrate the start of the famous bull-running festival with the annual launch of the chupinazo rocket in Pamplona. AP
"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 course through the narrow streets from a holding pen to the city's bull ring where the animals will be killed in a bullfight.
Revellers hold up their red scarves during the start of the San Fermin festival in Pamplona. Reuters
The bravest -- or most foolhardy -- festival-goers run as close as possible to the tips of the horns, hopefully without being gored.
The first bull run, which traditionally draws the largest number of participants, is on Sunday.
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.
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.
Revellers celebrate the start of the San Fermin festival in Pamplona. Reuters
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, which was 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).
A reveller is fed alcohol through a tube at the start of the San Fermin festival in Pamplona. Reuters