The nine-day street drinking party got under way at midday with the traditional shout from the city hall balcony of "Viva San Fermin!," followed seconds later by the firing of the firecracker known as the chupinazo. On Wednesday, daredevils will race just ahead of huge bulls running along Pamplona's cobblestoned streets, and gorings are virtually assured.
The rocket blast was the signal to the revelers — almost all in the festival's traditional dress of white shirts and pants with red sashes — to erupt into the party mode that dominates Spain through August. People sang and whooped while drenching each other with sangria, cheap wine and champagne.
Though the tourist count is down in Pamplona this year and merchants are complaining of low sales as Spain struggles with the European debt crisis and 20 percent unemployment, that didn't matter to David Marcilla after a woman dumped a huge bucket of water on top of him from her terrace in a third-floor apartment.
"There's a crisis in Spain, but there isn't today in Pamplona and everyone is partying," said Marcilla, 16, his white shirt already stained pink by wine.
The party's start was marred when Basque separatists prohibited from displaying their flag on a huge stage raised one into the air from the crowd and unfurled a banner demanding that terrorists convicted of bombings and killings be moved to prisons closer to their relatives.
The event witnessed by an AP photographer prompted police officers to move into the sea of people packing the plaza, swinging their batons. Fistfights also broke out between supporters of the ETA Basque separatist group and Spaniards opposed to the Basque independence movement.
Some in the crowd responded to the police action by hurling bottles of beer and champagne at officers, but some police hit people in the crowd, according to witnesses and photos by news photographers that showed the clashes.
"The pushing escalated and the cops started coming in and beating people with batons," said Daniel Woodfull, a 20-year-old Canadian university student from Windsor, Ontario.
New Zealand dairy farmer Helen Campbell, 23, said she was hit by a bottle in the head as riot police pulled people from the crowd, but she was uninjured.
An Associated Press reporter saw officers removing the Basque flag from the plaza, and four carried out one man who appeared to be injured or unconscious.
Police at the scene declined comment, and Pamplona police spokesman Antonio Iberni said he was unaware of any disturbances or injuries. He added that if people had attempted to raise a banner in support of ETA prisoners, officers would have waded in to remove it.
But the party resumed quickly after the melee, with television images showing masses of red and white dressed partiers swaying back and forth in the square and roaring "San Fermin!" and "Ole!" as the rocket blasted off.
"There's so much wine on the ground you could get drunk by drinking it off the floor," said Samantha Arnold, a 25-year-old pharmaceutical saleswoman from Australia.
In an almost laughable bid to try to keep the noise level down during the fiesta known for 24-hour street partying, Pamplona town hall on Monday banned street vendor sales of vuvuzelas, the droning plastic horns so popular at World Cup matches in South Africa.
San Fermin's first bull-run starts at 8 a.m (0600 GMT) Wednesday, when hundreds of people race ahead of six fighting bulls and six bell-tinkling steers — meant to keep them in a tight pack — that charge down the 930-yard (850-meter) course from a holding pen to the northern town's bull ring.
In the evening, the bulls will be killed in the bull ring, and their meat gets served up in Pamplona's restaurants.
Dozens of people are injured each year in the morning runs. Most get hurt after falling, but some are gored and trampled by the beasts.
Last year's festival saw the first goring death in nearly 15 years.
The fiesta became a big international event after Ernest Hemingway wrote about it in his 1926 novel, "The Sun Also Rises."
— Associated Press writers Ciaran Giles and Victor Caivano contributed from Madrid and Pamplona.
