TOMBSTONE, Ariz. —  Volunteers who have converged on the Mexican border to watch for illegal immigrants are disrupting U.S. Border Patrol (search) operations by unwittingly tripping sensors that alert agents to possible intruders, an agency spokesman complained Monday.

Scores of participants in the Minuteman Project (search) began assembling late last week and planned to begin regular patrols on Monday, in an exercise some law enforcement authorities and civil rights groups fear will result in vigilante violence. Many of the volunteers were recruited over the Internet, and some planned to be armed.

Over the past few days, they have set off sensors, forcing agents to respond to false alarms, said Supervisory Border Patrol Agent Jose Maheda.

"Every sensor has to be addressed," Maheda said. "It's taken away from our normal operations."

The volunteers planned to start fanning out Monday across 23 miles of the San Pedro Valley (search) to watch the border and report any illegal activity to federal agents.

The idea, according to project organizers, is partly to draw attention to problems on the Arizona-Mexico border, considered the most porous stretch of the 2,000-mile southern border. Of the 1.1 million illegal immigrants caught by the Border Patrol last year, 51 percent crossed into the country at the Arizona border.

Jim Coniglio of Tucson, who plans to patrol with other volunteers this week, said residents in some areas of the border have complained of being "overrun routinely" by migrants. "They're feeling insecure," he said.

The Border Patrol opposes the operation. "The possibility for something going drastically wrong is very high," Maheda said.