The ritual, called Tinku, a word that means “encounter” in both Aymara and Quechua, was once widespread throughout the Andean world, predating the arrival of the conquistadors. Anthropologists say it now tenuously exists just in this isolated pocket of Bolivia, seven hours southeast of La Paz by bus on a washboard dirt road.

To the chagrin of Roman Catholic priests who would like to see Tinku fade into the past, political officials here want it to survive.

“Tinku is a sublime, beautiful act,” said Wilson Araoz, the mayor of Sacaca and a leading official in the Popular Indigenous Movement, a party that is part of the coalition supporting Evo Morales, Bolivia’s first indigenous president.

Emboldened by Mr. Morales’s efforts to strengthen Bolivia’s indigenous cultures, Mr. Araoz’s party is one of several political organizations pushing to preserve endangered traditions like Tinku.

“There’s a predisposition in all of us to do Tinku,” Mr. Araoz said in an interview. “I believe that denying that impulse is harmful.”

No one disputes that Tinku can be harmful, at least physically. The fighting, though ritualized through music and dance, is far from organized, less like boxing and more like street brawling.

Not everyone takes part. Men of roughly equal size and age square off against each other on the streets surrounding the plaza, though sometimes women also enter the fray. Some of the men wear leather helmets and gloves and carry woven coca-wallets. But the fighting can also be bare knuckle.

In one fight at a dirt intersection, two men in their 50s punched and kicked each other for about 10 minutes, their faces bloodied as a crowd cheered them on. By the time exhaustion overwhelmed them, both were still conscious, though in a trancelike state.

Bystanders sometimes step in to break up contests that become too lopsided. In addition to bruised faces and limbs, deaths sometimes occur.

No one died at this year’s Tinku here, but blood certainly flowed. Some of the fighting evolved into generalized rock-throwing brawls with screams of aggression in Quechua and Aymara.

Some of the Tinku fighters could be found passed out on the ground, though it was not clear whether concussions or chicha were to blame.

“It was a good year, but slower than before,” said José Acuña Gabriel, 22, a Quechua-speaking Indian who wore a colorful ceremonial vest over a secondhand Operation Desert Storm T-shirt.

Expressing concern with Tinku’s violence, priests in this region, North Potosí, have been trying to eliminate the ritual fighting. Some priests have told indigenous leaders that church aid and community religious services could be withheld if residents took part in Tinku.

“There are profound things within these people that I still don’t understand,” said Carlos Ortigosa, a Spanish priest who has lived in Sacaca for the last 10 years. “But basically Tinku is an event in which people kick each other when they’re down and die with some frequency. It’s not agreeable to see people treating each other so badly.”

Anthropologists say Tinku represents much more than fighting. But the fighting, often between members of different communities, can be a way to confirm or defend collective landholdings, or to bring good fortune at harvest time.

It is also a chance for young men to show off in front of women from other communities. Couples often meet at Tinkus and marriages are known to result.

Tristan Platt, director of the Center for Amerindian Studies at the University of St. Andrews in Scotland, said Tinku’s origins were hard to pin down though indigenous communities might have practiced something similar as far back as 1100. The periodic channeling of communal violence may serve to reduce conflict during the rest of the year, he said.

Rare acts of cannibalism have occurred, Mr Platt said, and are thought by participants to be legitimized by the Catholic eucharist, letting them believe they are eating the flesh of a divinized human being.

Ramiro Molina, an anthropologist and director of the Museum of Ethnography and Folklore in La Paz, said blood was required for Tinku’s participants to feel they had consummated a harvest fertility rite.

“The West isn’t used to seeing such violence out in the open, ritualized and organized in such a way,” Mr. Molina said. “So of course this seems shocking at first glance.”

Most of the several hundred people who attended were subsistence farmers who walked to Sacaca and bartered in the town plaza for corn, blankets, shoes, chicha and coca leaf before the dancing began. Those who took part said the fighting was as natural as farming potatoes.

“I do it because it’s fun,” said Esteban Lisidro Aguilario, 20, between turns playing the Tinku dance’s haunting rhythm on his pinquillo, a flute made for the occasion.

Sacaca, even with its isolation, feels the tug of the outside world in ways that are diluting the appeal of rituals like Tinku. Entel, the national phone company, opened a booth here five months ago with three Internet-connected computers. Many of the town’s children and teenagers seemed more entranced by the instant messaging and games on those machines than with the centuries-old ritual unfolding around them.

This year’s Tinku drew only about half the number that had come three years ago.

Bolivia’s largest Tinku is not in Sacaca but in Macha, another town in North Potosí, where thousands gather to fight each May. The police there sometimes disperse brawls with tear gas. Officials in Macha have started charging admission to foreign tourists.

Sacaca’s leaders hope their ritual may become an attraction as well, though for now outsiders are still oddities. The only lodging here is a room in a shopkeeper’s house on the plaza for $3 a night.

“Tinku is like a psychologist that helps us overcome our traumas,” said Osvaldo Echeverría, a Sacaca cultural official. “We have a lot to learn about the world, but the world also has a lot learn from us.”