The triple-strand wire, meant to keep smugglers from attacking agents, will stretch five miles when completed this summer -- the longest expanse of this type of wire ever used on the Southwest border.
Federal authorities in the past have avoided using fortifications with such negative symbolism. Hundreds of miles of barriers going up in other areas have had to meet "aesthetically pleasing" federal design standards.
Critics say the new approach is inhumane and could leave illegal immigrants bloodied.
Border officials in San Diego say it was necessary and already is proving effective.
They say they opted to augment the existing fencing with razor wire amid escalating violence across from Colonia Libertad, one of Tijuana's most notorious smuggling enclaves.
The hilly area, roughly between the San Ysidro and Otay Mesa ports of entry, is already one of the most heavily fortified along the Southwest border, with primary and secondary fences, stadium lighting and camera towers.
The area has been the scene of frequent clashes between rock-throwing youths and agents firing pepper spray and tear gas. Despite using tear gas to disperse attackers and improving cooperation with Mexican authorities, U.S. authorities are still being attacked, said San Diego's Chief Patrol Agent Michael J. Fisher.
Fisher drew criticism late last year after his agents began tear-gassing densely populated areas, sending some residents to a hospital. He said the safety of his agents is his top priority.
"We didn't just decide to put up concertina wire," Fisher said. "This is a 1.5-year process on . . . how to make the border safe and secure, and to keep our agents safe from assaults."
So far, about a mile of the concertina wire is up. The installation started in December for a 60-day test period and was expanded in February. The wire runs atop the secondary fence, which sits roughly 50 yards inside the primary fence.
According to the Border Patrol, there were half as many assaults in the five months since the wire went up -- 58, compared with 122 in the five months before the installation.
Illegal entries were also down more than 50% over the same period, from 16,322 to 6,319, according to Border Patrol statistics.
Immigrant rights groups call the new fencing a major move toward border militarization. They say it's not the right solution.
"This is a primitive way of trying to conduct border enforcement," said Pedro Rios, San Diego area program director for the American Friends Service Committee. "Razor-sharp wire isn't going to stop people from coming across, and I doubt it will stop people from throwing rocks."
Enrique Morones, president of the Border Angels, a San Diego-based immigrant rights group, said he thought it would push more illegal immigrants into dangerously remote desert and mountain areas.
"This fence shows a total disregard for human life," said Morones. "What's next? Gun racks with machine guns and bazookas?"
The new tactics are praised by foes of illegal immigration, who frequently call for stricter enforcement. "This is a fence that is in plain view," said Steven Camarota, director of research for the Center for Immigration Studies, which favors curbs on immigration. "One has to keep in mind that the illegal alien trying to enter the U.S. is responsible for his own actions."
Border Patrol officials said they have tried to minimize the dangers of the new wire by lighting the area at night and erecting warning signs in Spanish. They predict that the wire will reduce injuries by serving as a deterrent. In the past, many immigrants have injured legs and ankles jumping from the 17-foot-high barriers.
Since the first concertina wire went up, only one immigrant has been injured by it, officials said. He got tangled up in the wire and was treated for minor injuries and returned to Mexico, officials said.
Concertina wire and regular barbed wire have been used on a limited basis in the past, mostly to fortify ports of entry and other federal facilities along the border. Most of the border fencing going up this year is made of welded wire or steel tubes without any sharp edges.
Fences topped with concertina wire have proved tough to breach, authorities say. Smugglers have tried without success to rip down the new wire by chaining it to a moving car.
Fisher, the Border Patrol chief, said he won't give an inch.
The fence, he said, sends a strong message to smuggling groups that threaten his agents:
"It will be a long battle unless we can show the smugglers that we won't cede the area. Nor will we retreat our position further to the north, and allow them to operate with impunity."