Tombstone, Ariz. ? Hundreds of volunteers, some of them armed, are expected to take up positions along the Mexican border Friday and begin patrolling for illegal immigrants — an exercise some fear could attract racist crackpots and lead to vigilante violence.

Organizers of the Minuteman Project said the civilian volunteers, many of whom were recruited over the Internet, will meet first for a rally in this one-time silver mining town, then fan out across 23 miles of the San Pedro Valley to watch the border for a month and report sightings of illegal activity to Border Patrol agents.

Minuteman field operations director Chris Simcox described the project as “the nation’s largest neighborhood watch group” and said one of the goals was to make the public aware of how porous the border is.

Jim Gilchrist, a retired accountant from Aliso Viejo, Calif., who organized the project, said that some volunteers will carry handguns, which is allowed under Arizona law, but are being instructed to avoid confrontation, even if shot at.

Still, law enforcement officials and human rights advocates are worried about the potential for bloodshed.

Critics contend the project may attract anti-immigrant racists and vigilantes looking to confront illegal immigrants. At least one white supremacist group has mentioned the project on its Web site.

Michael Nicley, chief of the U.S. Border Patrol’s Tucson sector, said the volunteers are “not the kind of help the Border Patrol is asking for.”

Cochise County Sheriff Larry Dever said he fears immigrant smugglers might open fire on the volunteers.

“I wouldn’t anticipate that people of that persuasion would act or react any differently to anybody, citizen or law enforcement alike, if they were confronted and felt like their cargo was in jeopardy,” he said.

The project’s organizers gave assurances the volunteers would be closely monitored. “If it gets to a situation where someone’s life is in danger,” said David Helppler, Minuteman security coordinator, “I will end the project.”

Project organizers said they expect 800 to 1,000 volunteers. How many might actually show is unclear; similar efforts in the past few years flopped. One of them drew only about a half-dozen people.

On Wednesday, the Homeland Security Department announced that it is assigning 534 additional agents to the porous Arizona border to help keep out potential terrorists and illegal immigrants.

The 370-mile Arizona border is considered the most vulnerable 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.