In Photos: Pamplona's bull running festival kicks off with a spectacular display of color

Thousands of revelers from all over the world packed into Pamplona's main square to mark the start of the famous bull-running festival on Thursday. The San Fermin festival kicked off with the traditional midday launch of a firework rocket—known as the Chupinazo—from Pamplona's Town Hall balcony, decked out with the Spanish and Basque flags.

The crowd in the square below raised their hands and screamed, "Viva San Fermin!" as a marching band made its way through the middle of the throng. This was the signal for the revelers to start dousing each other with red wine and sangria. Those lucky enough to occupy the balconies around the square threw buckets of water or more red wine onto the crowd below.

The square was filled with people waving red neckerchiefs and wearing the festival's traditional outfit of white shirts and trousers, though they did not stay white for long once the sangria started flying.

The Chupinazo is held a day before the first of eight bull runs. Thousands of people at the festival test their speed—and bravery—by racing ahead of six fighting bulls along an 850-meter course, from a holding pen to the city's bull ring. The bulls are then killed by matadors in bullfights each afternoon.

The nine-day fiesta was immortalized in Ernest Hemingway's 1926 novel The Sun Also Rises and now attracts more than a million foreign tourists each year. Last year, 1.5 million visitors were drawn to the famous event.

However, the colorful festival has its dark side. Dozens of people are injured each year in the runs. Last year's event was also marred by widespread reports of sexual harassment and assault. Police arrested 16 men in relation to five violent attacks. "No celebration should mean women's rights are suspended, nor institutional responsibility," said Vanesa Eguiluz, the local councillor for equality. "A city is only free when women can move around it with freedom, without fear or violence."