“It’s a real shame,” Mr. Fisher said, gazing down as a young boy playing on the beach darted briefly across the border, then back again. “It is a nice area with the historical marker. Having people meet and mingle is good. But unfortunately, any time you have an area that is open, the criminal organizations are going to exploit that.”

“We cannot,” he added, “have it open, not at the expense of reducing the ability to patrol the border.”

The new fencing is part of a 14-mile project to reinforce and build new barriers from the ocean to areas east of the Otay Mesa port of entry. The project includes filling in a deep valley known as Smuggler’s Gulch, a notorious crossing point just east of the park, with tons of dirt, to the dismay of environmentalists.

Unlike the trend in the past year or two along most of the 2,000-mile Southwest border, Mr. Fisher said, illegal crossings have increased in the San Diego area, along with attacks on agents who encounter smugglers raining stones and other objects on them and their trucks. One-fourth of all such assaults, he said, occur in the San Diego sector, which more than a decade ago was one of the hottest spots for illegal crossings.

While a flood of new agents and bolstered fencing has pushed much of the crossings to the eastern deserts and the sea, where smuggling by boat is a growing problem, people still regularly climb over, tunnel under or cut through the fence, sometimes with blowtorches and sophisticated cutting tools.

But critics of the plan to extend the fencing in Friendship Park said the Border Patrol had exaggerated problems there, one of a smattering of spots along the border where the prospect of new fencing has dampened cross-border bonhomie.