By RANDAL C. ARCHIBOLD and JULIA PRESTON

NACO, Ariz. — As the Department of Homeland Security pushes to complete 670 miles of fencing along the Mexican border by the end of this year, it is confronting the sharpest resistance yet while conceding that physical barriers alone do not stop illegal crossings.

In the latest challenge, the Texas Border Coalition, an organization of mayors, county commissioners and economists opposed to the fence, filed a federal lawsuit on Friday. It says that the homeland security secretary, Michael Chertoff, failed to conduct required negotiations with property owners and local authorities when he ordered that the barrier be built in Texas. The group wants the construction halted.

The protests come as known efforts at illegal crossings — measured by the number of people detained at the border — have fallen 17 percent this year, after declining 20 percent in 2007, figures that Chief David V. Aguilar of the Border Patrol points to as proof that the overall approach to border enforcement is working.

Still, Mr. Aguilar and other officials acknowledge, the new fencing has mainly proved useful when it has been backed up with other enforcement methods, like electronic surveillance and aggressive prosecution of illegal immigrants caught by the Border Patrol.

Since last year, the steepest drops in illegal crossings along the 2,000-mile border were recorded here in eastern Arizona and in places in Texas where those combined tactics were applied, official figures show.

Technical glitches have plagued plans to expand and enhance the electronic surveillance into a virtual fence, and it remains uncertain when it will be in broader use.

After months of delays, the Border Patrol approved a pilot system in February from its contractor, Boeing, which now says that most of a prototype tested along 28 miles of border will be replaced. The company will also build a virtual fence on another 30-mile stretch of southern Arizona and test similar technology on the Canadian border near Detroit this year.

Mr. Chertoff acknowledged in an interview that constructing physical barriers — as of last month, about 309 miles of fence had been built — is not the key to stopping illegal immigration, but he defended the fence’s usefulness.

“I don’t believe the fence is a cure-all,” Mr. Chertoff said. “Nor do I believe it is a waste. Yes, you can get over it; yes, you can get under it. But it is a useful tool that makes it more difficult for people to cross. It is one of a number of tools we have, and you’ve got to use all of the tools.”

As many as 2,000 immigrants a day still cross the Southwest border illegally, according to estimates by scholars well versed on the border. Continuing a decades-old cat-and-mouse game, the crossers move away from areas where the Border Patrol establishes control to more vulnerable points, most recently near San Diego.

In addition to the border enforcement, immigrant traffic is influenced by a variety of social, political and economic factors; the recent drop in known crossings, for example, occurred as the economy began to sputter, drying up construction jobs and others that lure immigrants.

Trinidad Alamea, who operates a small shelter across the border from Naco, said several nights had passed recently without a single immigrant seeking help. But, Mr. Alamea said, such fluctuations have occurred before, only for immigrant traffic to return when smugglers adjust to whatever new tactic or conditions confront them on the American side.

“The people are going to cross, wall or no wall,” he said.

Opposition to the fence intensified last month after Mr. Chertoff used authority provided by Congress to waive more than two dozen environmental laws and others to push ahead with construction. Mr. Chertoff said his department needed to bypass the laws if it was to meet the goal set by Congress two years ago of completing at least 670 miles of fence by the end of this year.

Fourteen United States representatives, all Democrats, including Bennie Thompson of Mississippi, chairman of the Homeland Security Committee, have said they support a lawsuit filed in April by two environmental groups — the Sierra Club and Defenders of Wildlife — that challenges Mr. Chertoff’s waiver.

Meanwhile, officials in border cities, Indian groups and educational institutions have stepped up their criticism of the fence. The broad array of dissenters are protesting what they say are the economic and environmental impacts of the fence for border cities, ranches and natural areas, and questioning whether the estimated $2.1 billion for its construction is the best use of border security money.

“No thought was given methodically to this idea of the fence,” Patricio M. Ahumada Jr., the mayor of Brownsville, Tex., said on Friday in Washington, where the Texas Border Coalition filed its lawsuit. “Homeland Security is using it to give a false sense of security to middle America that it will keep illegal immigrants and terrorists out, but it just isn’t true.”

At a Congressional hearing this month in Brownsville, the president of the University of Texas campus there, Juliet V. Garcia, testified that she had not been consulted before Homeland Security officials announced plans to build an 18-foot-high barrier that she said would leave the campus’s technology center and golf course “on the Mexican side of the fence.”

Ms. Garcia said she believed that the fence would pose “serious harm to the university on many fronts,” including risks to students’ safety, to its property investments and to its educational mission.

In Naco, historically one of the more troublesome spots for the agency in its campaign to curb illegal crossings, construction workers have clawed a trench and fired up blowtorches as they maneuvered giant grates of mesh into place for a second row of fencing. A hodgepodge of steel plating, heavy-duty mesh and towering metallic slats now extends for miles, replacing ragged, five-strand barbed wire fencing that for decades marked the Mexican border.

Arrests of illegal immigrants are down. And this village’s larger twin in Mexico has seen its economy, dependent largely on smuggling, plummet.

Border Patrol officials interpret both developments as signs of success. Michael Hyatt, a Border Patrol field supervisor, noted that a number of steps had been taken in this area, aside from the fence, to frustrate smugglers and repeat crossers, an acute problem here.

Although Mr. Hyatt would not disclose how many agents work there, a station built for 26 agents will be replaced next year by new quarters housing 450. Altogether, he said, word is spreading that Naco is not as easy as smugglers make it out to be.

“They see some of the infrastructure we’re putting out there,” he said.

But Mr. Hyatt is not raising a toast yet. The fence, he said, “is not going to stop people from coming across.”

“It is going to deter some from crossing,” he said. “It’s going to relocate some to other areas to cross where there isn’t a fence.”

Officials of the Homeland Security Department give a broad estimate of $3 million per mile to build the fence, or about $2.1 billion to reach the goal this year, out of $5.2 billion for the Border Patrol this year. The officials have declined to provide Congress with a more exact price tag, saying costs vary depending on the difficulty of the terrain.

Under perhaps the most effective program, which is used in limited areas of Arizona and Texas, federal prosecutors press criminal misdemeanor charges against immigrants caught by the Border Patrol, putting them in detention for up to two months, well beyond the several hours they normally would be held before being returned to Mexico.

The Justice Department is adding 64 federal prosecutors along the border to support these criminal cases. But the authorities cannot expand the program much further because of lack of detention space.

In the Border Patrol sector around Yuma, Ariz., apprehensions dropped 69 percent since the program was begun last year, official figures show. The agency uses its apprehension figures as an approximate gauge of illegal crossings, assuming that agents make fewer arrests when fewer immigrants try to cross. In March 2007, agents caught 5,571 in the Yuma sector; this March, they caught 751.

In a new survey of immigrants from the south-central Mexico farming state of Oaxaca, Wayne A. Cornelius, director of the Center for Comparative Immigration Studies at the University of California, San Diego, found that they believed the border had become much more dangerous to cross illegally. The percentage of those caught at least once by the Border Patrol had doubled since 2005, to 48 percent from 24 percent, according to the survey, which was conducted among 821 immigrants from December 2007 to February.

But 97 percent of the immigrants who set out to cross the border illegally said they had succeeded eventually, Mr. Cornelius found, a figure that has not decreased since 2005. In the survey, 82 percent of the immigrants who succeeded in crossing said they came through San Diego. In fact, overall apprehensions in San Diego — where fences have been in place since 1993 — defied the downward trend, jumping by 20 percent in 2007, Border Patrol figures show.

Meanwhile, groups opposing illegal immigration are also protesting the fence construction — in their case, because of unhappiness with its slow progress. A new group in Tucson called Techno Patriots has set up several thermal imaging cameras at the border to watch for illegal crossers by laptop computer from their homes.

The Cochise County Militia in Naco, one of several civilian patrol groups along the border, is making plans to bring in dozens of volunteers in the coming months to help apprehend illegal immigrants who the group says still manage to get in despite the new fencing.

Bill Davis, the group’s leader, called the barriers “a joke.”

“This is still the worst 90 miles of border,” Mr. Davis said.

Randal C. Archibold reported from Naco, Ariz., and Julia Preston from New York.