SAN YSIDRO, CALIF. -- Muriel Watson watched as three crouching men made their way warily across a field of weeds in midafternoon about a half-mile from the border with Mexico. After lying low while a dozen U.S. Border Patrol vehicles drove by a few yards away and an observation helicopter flew overhead, the men ran across the road and disappeared into brush on their way toward a U.S. highway.

"We're a sovereign nation, but we don't know who they are or where they're going," said Watson, the widow of a Border Patrol agent.

She said she had pointed out the men to the passing Border Patrol officers, but they were too busy chasing larger groups of illegal aliens, or their vehicles were already full of people rounded up trying to beat the evening rush hour, when thousands would pour across the border under cover of darkness.

"I feel like Alice in Wonderland," Watson said as she stood about 50 yards from where the men had crossed the road. She gazed at the Mexican border. "And the people who don't want to put a stop to this are like the Mad Hatters."

The scene was a section of U.S. land just west of the San Ysidro border crossing. Residents of nearby communities complain that the United States has lost control of a swath of its territory and that a surge in border violence has made the area a volatile, dangerous no man's land. They worry that the violence will soon affect them directly, as it is already terrorizing the illegal migrants who cross the border in search of jobs.

Led by Watson, they have launched a campaign dubbed Light Up the Border, in which they line up their cars by the hundreds with headlights beaming across a stretch of vacant land toward Mexico. The monthly demonstrations, which started in November, have prompted denunciations in Mexico, charges by Hispanic groups of racism and an increase in border tensions. Counter-demonstrators started showing up in April, holding up mirrors to reflect the lights back at the protesters.

Although undeterred by the criticism, Watson this month decided to suspend Light Up the Border because of an unhealthy combination of mosquitoes and white supremacists. In addition, she said, police are promising concerted action to tackle the problem.

San Diego County health authorities recently warned of outbreaks of malaria and encephalitis from mosquitoes that breed in the waters of the Tijuana River, which carries raw sewage from Mexico through the area where the demonstrations have been held. "There's no way I can invite 1,500 people down to a place where they can be gnawed on by mosquitoes," Watson said.

Then a group calling itself the White Aryan Resistance issued a letter threatening to organize fellow "skinheads" to wage "war" against counter-demonstrators and illegal immigrants. Watson said she was "appalled" by the letter and wanted no part of the group.

In recent days, the San Diego police department has pledged to assign a SWAT team and an elite unit called the Special Response Team to help patrol the area west of the San Ysidro crossing. Wednesday, the Border Patrol announced it was installing floodlights in the area and assigning 30 more agents there in a bid to curb what it called "an alarming increase in violence."

Since January, the Border Patrol said, seven migrants have been killed and at least two wounded in the no man's land west of existing floodlights, compared to five such killings during all of last year. The Border Patrol blames six of the deaths on "border bandits," gangs of Mexican criminals who ambush their countrymen on U.S. soil, then flee back into Mexico.

The bandits commonly attack one member of a group of illegal migrants to intimidate and rob the rest of the cash they usually carry to pay alien-smugglers or start new lives in the United States, Border Patrol spokesman Ted Swofford said. "They don't need a reason to initiate violence. They do it just to get everyone's attention."

Dozens of other people have been killed or injured by cars while trying to cross a Mexican toll road that runs along the border on the Tijuana side and on two U.S. interstate highways that converge on the legal crossing point from the north.

In Mexico, politicians and the press tend to blame the violence and deaths on the Border Patrol and angrily denounce any increased vigilance as "militarization" of the 2,000-mile frontier. U.S. officials charge that Mexican and U.S. critics focus on incidents in which Border Patrol agents have shot illegal aliens and largely ignore the rape, robbery, murder and mayhem committed increasingly on the border by and against Mexicans.

In a report issued this month, the U.S.-based human rights group Americas Watch charged that "United States border control agents have committed many serious abuses against Mexican nationals" in trying to curb drug smuggling and illegal immigration.

But, said Border Patrol spokesman Swofford, from January through mid-May, there have been 73 assaults on agents, an increase of 145 percent over the same period last year. More than 100 stoning incidents also have been reported, resulting in injuries to several agents, he said.

Swofford attributed the increased violence to "the changing profile of the average alien." Compared with five years ago, the typical illegal migrant is now younger -- average age 20 -- and from an urban rather than rural background. Aliens now are prone to be "more aggressive," he said.

In the first four months of this year, apprehensions averaged 1,300 a day in the San Diego-Tijuana sector, up more than 50 percent from last year, Swofford said. The Border Patrol estimates it catches one of three who come across, but some agents put the figure at closer to one in eight.

According to Jorge Bustamante, a Tijuana-based academic who monitors Mexican migration, the latest clamor on the U.S. side about controlling the border smacks of hypocrisy. "For us, this thing about the U.S. losing control of the border is kind of a joke," he said. "They don't want to have control of the border. It would be uneconomic."

Instead of a "criminal phenomenon that has to be solved through police measures," the migration of undocumented Mexican workers across the U.S. border "is a labor-market phenomenon where the demand in the United States is as real as the supply," Bustamante said.

Countering U.S. charges that such illegal immigration drains social services, he argues that Mexican workers pay hundreds of millions of dollars a year in Social Security taxes, helping to finance the retirements of Americans.