Phoenix ? Volunteers for an effort to patrol the Mexican border reported their first sighting of suspected illegal immigrants, resulting in 18 arrests, authorities said Sunday.

Participants in the Minuteman Project spotted the migrants Saturday near Naco as the volunteers were surveying the border to familiarize themselves with area. When agents arrived, they apprehended 18 people, Border Patrol spokesman Andy Adame said.

“You observe them, report them and get out of the way,” said Mike McGarry, a spokesman for the project.

The volunteers reported another illegal immigrant after he wandered onto the campus of a Bible college near the community of Palominas, where about 100 Minuteman participants were staying.

The man walked in and said he needed food and water. Volunteers helped him and notified federal agents, who picked him up, McGarry said.

The man was weary from traveling but did not need medical attention, Adame said.

Minuteman volunteers planned to start regular patrols today, fanning out across 23 miles of the San Pedro Valley to watch the border and report any illegal activity to federal agents. It’s an exercise some law enforcement authorities and others fear could lead to vigilante violence.

McGarry said about 200 people would be in place for today’s patrols, although human rights activists and some authorities have questioned whether the project will attract as many volunteers as organizers expect.

Law enforcement officials said the volunteers were keeping the peace, despite concerns they might become confrontational with immigrants.

The Arizona-Mexico 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.