A red-and-white sea of revelers soaked each other with wine in a packed Pamplona square on Sunday to kick off Spain's most famous fiesta, the San Fermin bull-running festival.

A shout from the City Hall balcony of "Viva San Fermin!" and the lighting of a firecracker known as the "chupinazo" at noon (1000 GMT) set off the bedlam, which marks the official start of the nine-day street party.

Masses of fun-seekers from around the world squeezed into the square under an overcast sky cheered, danced and sprayed each other with sangria and cheap wine, tuning white shirts to pink.

The crowds dressed in traditional white outfits and red neck scarves passed large yellow and white inflatable balls over their heads as scores looked down from crowded apartment balconies.

Sam Madden, a 26-year-old electrician, came from London with a friend to take part in a bull run.

"We don't know what to expect, if we are going to die or what. It's cool, it's going to be crazy. We know it can be dangerous but we have to do it for a bit of adrenaline," he said.

The festival in honour of Saint Fermin, the first bishop of Pamplona, dates back to medieval times and it features religious processions, folk dancing, concerts and round-the-clock drinking.

But the highlight is a bracing, daily test of courage against a thundering pack of half-ton fighting bulls through the city's cobbled streets.

Each day at 8:00 am hundreds of people race with six huge bulls, charging along a winding, 848.6-meter (more than half a mile) course through the narrow streets to the city's bull ring, where the animals will be killed in a bullfight.

The first bull run, which traditionally draws the largest number of participants, is on Monday. A run takes on average just under four minutes.

Last year 50 people were taken to hospital at the festival's eight runs, including an Australian woman who was gored by a bull.

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