Under cover of darkness, a steady drizzle and a patchy fog, Manuel Parra-Sierra and 11 other Mexicans scaled the eight-foot-high fence and bolted into the muddy salt flats of Border Field State Park.

Their destination: Imperial Beach, a community two miles away that illegal Mexican immigrants have long used as a staging point en route to other places in California and across the United States.

But this was not their lucky night. Almost immediately, the group was detected by seismic sensors, then tracked from a hilltop more than a mile away by a Border Patrol agent operating a Loris infrared night-vision scope. By radio, the officer directed colleagues in Ford Broncos to the scene about three-quarters of a mile north of the border. The illegal crossers were then quickly surrounded by agents shouting "halt" in Spanish.

"It's the economic situation we're living through," said Parra-Sierra, 33, explaining why he left his wife and two children at home in Yucatan state, where he earned less than $34 a week as an electrician, to seek work in the United States.

The group was part of a wave of illegal border crossers driven by Mexico's worst economic recession in 60 years, an influx that is being met by a major new U.S. effort to tighten control of the southwestern border. Launched Jan. 16, the program reinforces Southern California and Arizona with 300 Border Patrol and immigration officers supported by police, National Guard and military personnel. The move serves to bolster President Clinton's claims of getting the border under control and ending the "neglect" of previous administrations that had made the San Diego area a freeway for illegal immigration.

The effort also carries high stakes politically in this election year -- especially in California, a crucial constituency in which illegal immigration is a key issue. At the same time, it has provoked expressions of alarm and condemnation from officials in Mexico, who accuse the United States of "militarizing" the border.

An existing campaign, "Operation Gatekeeper," already has made crossing here more difficult. Notably, it has driven up the costs that smugglers charge to guide illegal aliens through the human, physical and technological barriers that have changed the face of the border in the last couple of years.

As a result, illegal crossers have resorted increasingly to document fraud at legal ports of entry, and many have moved eastward into rougher and more dangerous terrain.

On Jan. 20, one young Mexican was killed and five others were injured, one critically, when they fell off a 120-foot cliff in the Otay Lakes area about 20 miles east of Imperial Beach. The survivors were rescued by the Border Patrol and a local sheriff's department helicopter.

There also has been a change in the type of people coming north illegally, according to U.S. and Mexican authorities. Traditionally, most illegal crossers have been jobless laborers or farm workers from a few states in Mexico. Now, as in a previous economic crisis in the mid-1980s, they come from all over the country and include a higher proportion of skilled workers and middle-class people. The latest crossers have been fleeing an economy that shrank 6 percent and lost 1 million jobs last year while running 50 percent inflation and sustaining a nearly 60 percent drop in the value of the peso.

"In the last couple of months we've been seeing people who would never have considered coming to the border before," said Johnny N. Williams, the Border Patrol chief for the 66-mile San Diego sector. "They tell us they're forced to try something."

Parra-Sierra seemed to be a case in point. "The money I earned was not enough to feed my family," he said glumly as Border Patrol agents put plastic handcuffs on his compatriots, a few of whom had removed their pants in preparation to cross a small channel. "I came with the idea of working in whatever job I could find."

Wearing a green jacket, bluejeans and muddy running shoes, the visibly apprehensive Mexican said his Jan. 16 foray was his first attempt to enter the United States, an assertion supported by a computer that found no record of an encounter with the Border Patrol in the recent past.

In January, apprehensions of illegal aliens along the entire 2,000-mile border with Mexico rose 66 percent from a year ago. In the San Diego sector, the most heavily traversed portion of the frontier, the Border Patrol last month caught an average of 1,890 a day, up 51 percent from January 1995, reflecting both the increased traffic and the Border Patrol's increased ability to catch people.

Efforts to step up enforcement along this stretch of the border began slowly in 1990 and then accelerated with the launch of Operation Gatekeeper in October 1994. Since then, illegal crossers in this sector have had to contend with 14 miles of fences made of surplus iron runway mats, a 50 percent increase in Border Patrol agents, newly bulldozed access roads for patrol vehicles, rows of stadium lights that illuminate smuggling routes, an enhanced array of night scopes, hundreds of seismic, metallic and infrared sensors, seven helicopters and a computerized identification system called "IDENT" that records the photographs and fingerprints of everyone apprehended.

"The scopes basically run the show at night," Border Patrol supervisory agent Ron Henley said after personally collaring two of the illegal crossers rounded up in Border Field State Park.

"They took advantage of weather they thought we were not going to work in," said Steve Kartchner, another agent who arrived on the scene. While fog tends to limit the scopes' ability to pick people out of the darkness, on a clear cold night "you can almost count fingers," he said.

"The weather is about the only thing that can defeat us," Henley said.

With the IDENT system, authorities have a new weapon against smugglers who guide groups of migrants across the border. Previously, they would give different names each time they were caught and claim to be ordinary migrants themselves, claims that could not be disproved. The Border Patrol, a branch of the Immigration and Naturalization Service (INS), would then bus them back to Mexico with other detainees, and the smugglers would be back in business.

Now, repeat offenders and previously deported criminal aliens can be more readily identified, then prosecuted on felony charges if they come back.

"For the first time, we have a border that is being managed," asserted Alan Bersin, the attorney general's special representative on border issues.

Fred Krissman, a researcher at the University of California-San Diego's Center for U.S.-Mexican Studies, charged, however, that the border-control program is merely "deflecting flows from one sector of the border to another." He added, "All we're really doing is raising the cost and difficulty of migration, but we're not even slowing, let alone stopping, the historical pattern."

The main effects of the campaign, he said, are to put on a political show and to warn America's already fearful illegal immigrants against asserting labor rights.

Migrants like 23-year-old Hector Camarillo Delgadillo express more immediate concerns. "For the last five months there has been no work in the fields, no manual labor, no nothing," he said. He left his village in Zacatecas state out of "extreme necessity," he said, only to be caught Jan. 18 and brought to a Border Patrol station to await repatriation. He said he plans to return to his village.

Alicia Corona Lopez almost made it. A $27-a-week secretary from Guadalajara, she used another woman's border crossing card in a look-alike scam to get by an inspector at the San Ysidro port of entry, but was arrested when an undercover Border Patrol agent spotted her with a smuggler on the U.S. side.

She was to pay a driver $500 when she got to Los Angeles, agent Tim Sheehan said. The driver was charged with alien smuggling, but could not be prosecuted immediately because of a lack of jail space, he said. The smuggler fled.

At the San Ysidro crossing, the world's busiest port of entry with 45 million crossings last year, smugglers often operate within 50 yards of the inspection windows, said INS district director Mark Reed. "The smugglers are continually prodding," he said. "This has always been their turf. They're not going to give it up easily."

As he showed a reporter where the smugglers regularly gather, three men were boldly climbing a 12-foot fence that runs perpendicular to the border a few feet inside U.S. territory. Reed sprinted over and ordered them to climb down.

"They see California as a dream, as the promised land," said the Rev. Gianni Fanzolato, an Italian priest who runs a shelter for migrants in the Mexican border city of Tijuana. But it is harder than ever to get there, he said, and the number of migrants who report being turned back is now around 80 percent, up 30 percent from last month.

"It's more difficult now, but we're going to keep trying to cross, because we don't have any other choice," said Enrique Gonzalez Garcia, 37, who left a wife and seven children in central Mexico. Gonzalez said he had worked illegally in Los Angeles for five years as a gardener and factory hand. But when he tried to sneak back into California in October, he said, he was caught and punched by two Border Patrol agents. Now he is trying again.

Jose Roberto Portillo, a 36-year-old Honduran, said he was treated worse than that by Mexican police, who detained, beat and robbed him four times on his way north. It has taken him eight months to reach Tijuana, he said, and he has been unable to talk to his wife and child since he left.

In diaries, Fanzolato has recorded the stories that migrants bring to the shelter, tales commonly marked by poverty, hardship and violence.

One youth said he and a friend were riding north on a freight train when they were joined by three men who wanted to rape them. The youth said he submitted, but his friend resisted and was murdered on the spot, then dumped off the train. At the shelter, the troubled boy tried to kill himself. Then one day he showed up with a pistol and said he was leaving to hunt down his friend's killers, the priest said.

Another young migrant, a 19-year-old from Michoacan named Omar, kept a diary while riding a freight train to the border town of Mexicali, Fanzolato said.

"I saw much perdition of minors and adults," Omar wrote. "I saw drug addicts, robbers, murderers, rapists, vagabonds and young boys and girls dedicating themselves to prostitution. I realized all that can happen when one leaves home, and I remembered what my mama said: Don't go, because the world is very dangerous.' " CAPTION: POLICING THE BORDER Illegal border crossings have increased sharply because of Mexico's recession. The rise is being met by a stepped-up U.S. effort to tighten control of the country's southwestern border. Apprehensions on the U.S.-Mexican border for January 1996: 169,463. For the same period in 1995: 102,037 (an increase of 66 percent). Apprehensions in the 66-mile San Diego sector for January 1996: 58,582. For the same period in 1995: 38,871 (an increase of 51 percent). APPREHENSIONS ALONG THE U.S.-MEXICO BORDER BY COUNTRY OF ORIGIN Fiscal years 1991-95 1991 Mexico

1,058,174 El Salvador

8,280 Guatemala

4,901 Honduras

3,133 Other

3,388 TOTAL

1,077,876 1992 Mexico

1,132,148 El Salvador

4,393 Guatemala

3,690 Honduras

2,697 Other

2,646 TOTAL

1,145,574 1993 Mexico

1,195,951 El Salvador

5,174 Guatemala

4,458 Honduras

3,536 Other

3,767 TOTAL

1,212,886 1994 Mexico

963,132 El Salvador

4,935 Guatemala

4,376 Honduras

2,963 Other

3,695 TOTAL

979,101 1995 and percent change from 1994 Mexico

1,256,455

(+30.5%) El Salvador

3,528 ( -- 32.2%) Guatemala

3,840 ( -- 12.2%) Honduras

4,031

(+36%) Other

3,536 ( -- 4.3%) TOTAL

1,271,390

(+29.85%) SOURCE: Immigration and Naturalization Service CAPTION: A U.S. Border Patrol officer uses an infrared night scope. CAPTION: Illegal immigrants run head-on into traffic on California's Interstate 5 in an attempt to avoid Operation Gatekeeper, a U.S. campaign to make border crossings more difficult. CAPTION: Mexican men seek food at a shelter in Tijuana, Mexico. Poverty has driven many to attempt crossing the border. CAPTION: Above, illegal immigrants attempt to scale a steel fence at the California border after being surprised by Border Patrol agents. Agent Jim Zimmerman, at left, helps a migrant out of the trunk of a car at the Interstate 8 checkpoint. At right, agent David Swan scans the Cleveland National Forest. He was relocated from the Canada/New York border to assist with the onslaught of crossings in the San Diego area.