By GINGER THOMPSON
 
MEXICO CITY, Aug. 7 - It's 6 p.m., the busiest time of night during the busiest time of the year at Benito Juárez International Airport.

The migration supervisor, Alberto Pliego, calls it Jumbo Hour. It looks a lot like a human salmon run.

Mr. Pliego has at least six 747's pulling in from Frankfurt, Madrid, Paris, Amsterdam and Vancouver, and five agents. Their job is to stand in front of the flow of passengers pouring from the planes and pick out which ones are tourists and which are migrants trying to get past them and get to the United States.

"A migrant who makes it past the airport today,'' Mr. Pliego said, "will be in Tijuana tomorrow, and probably in Chicago the day after that."

Migrant smugglers - whose business is worth an estimated $1 billion in this hemisphere, second in profits to drugs - do a brisk business at the airport, which receives about 10,000 passengers each day.

Mr. Pliego's suit and tie made him look a little too buttoned down to guard against some of this country's most unscrupulous criminal operations. But by the end of the night, he had stopped more than a dozen Brazilians who tried to enter Mexico as tourists, but lacked suitcases, hotel reservations or credit cards. He supervised the deportation of two undocumented Armenians. Three Guatemalans were caught trying to enter the country with false visas. And one of Mr. Pliego's agents caught four undocumented Chinese travelers lingering over soft drinks and sandwiches in an airport restaurant.

The agent spoke no Chinese. The Chinese spoke no Spanish. But in limited English, each side seemed to completely understand the other.

The agent speculated that the Chinese men were waiting for a guide to help them get past migration checkpoints.

The Chinese said they were hungry.

The agent asked the Chinese for their travel visas.

The Chinese said they planned to stay in Mexico for only one night.

The agent escorted the Chinese men back to the same airplane on which they had arrived, ordering them back to Amsterdam.

The Chinese boarded without putting up a fight.

The Mexican authorities report that a surging number of migrants from all around the world are traveling through Mexico to get to the United States. So far this year, Mexico has detained nearly 112,000 illegal migrants, compared with 150,000 in all of 2001. Authorities said they expected total detentions for this year to reach 200,000. The Mexicans are under tough pressure from the United States, which since Sept. 11 has feared that global terrorists could easily slip into Mexico and then cross into the United States.

The overwhelming majority of those detained are from Central and South America, authorities report. But there are also increasing numbers from as far away as Pakistan, Armenia, Bosnia and Herzegovina, Poland, Ethiopia and China.

The migrants often arrive at Mexico's main airports and then travel by land to the border. But illegal migration routes and methods are as diverse as the people who use them. On Wednesday, the Mexican authorities detained four Chinese migrants on a private jet that made an emergency landing for fuel in the southern state of Chiapas. The pilots reported that they had picked up their undocumented passengers in Caracas, Venezuela, and that they planned to deliver them to smuggling contacts at a small airport north of Mexico City.

At a migration detention center to the east of Mexico City holding 500 people of every background, each farmer, bricklayer, auto mechanic and accountant had an epic story to tell. The director of the center, Hugo Miguel Ayala, said the migrants came from more than a dozen countries.

Among them was a 35-year-old Ethiopian woman named Alemayehu, who said she traveled from her homeland to Egypt, Moscow, Havana and Nicaragua before boarding a bus bound for Mexico City, hoping to reach New York.

And there was Yu Youqiang, who left his wife and small daughter in Fujian, China, to seek work in New York. He said he traveled to Frankfurt, then to Mexico taking nothing but a backpack and travel instructions from a smuggler scribbled on a scrap of paper.

A 32-year-old vegetable vendor, Mr. Yu said he had made it all the way to the border before he was caught by the Mexican authorities in a town whose name he could not recall. He said he had paid smugglers $5,000 for help reaching the United States. Relatives, he said, had agreed to pay $25,000 more once he arrived in New York.

"We come through Mexico because it's cheaper," he said in English and through a translator. He said some Chinese migrants flew directly to the United States from Hong Kong. But false visas cost a lot. And entering the United States through an airport is much harder than entering through the border.

"They say that it's easy to get across," Mr. Yu said. "You just have to walk."

It's fairly easy for Brazilians to enter Mexico. They are among 46 nationalities that are not required to get Mexican visas. The Mexican authorities report that Brazilians are coming in droves, and heading straight for the United States.

Migrants' passage through Mexico, while not new, is surging. The country's border with the United States has long made it a natural transshipment point for all kinds of illegal trafficking including drugs, guns and migrants. But since the Sept. 11 attacks against the United States, migration has become a national security priority - and often the source of diplomatic tensions - for authorities on both sides of the United States-Mexico border.

The United States has pressed hard on the Mexican government to increase security at its airports and borders, and to crack down against the criminal organizations that smuggle migrants into the United States, arguing that smugglers - known in Mexico as coyotes - could be easily enlisted by terrorists.

Mexico, cash poor and rife with corruption, struggles to comply with its neighbor's demands.

In an interview, Interior Minister Santiago Creel said Mexico had made important strides in preventing this country from becoming a transshipment point for terrorists. In December, the government upgraded systems that track foreigners who enter and reside in Mexico. In the last two years, Mr. Creel said, the government has dismantled more than 10 important migrant smuggling organizations, including one that was led by some 44 migration agents and police officers. But so far, the authorities have not detained any suspected terrorists trying to enter the United States from Mexico.

Mexico seems burdened in the struggle. While illegal migration through Mexico has increased by 144 percent over the last year, authorities said, the National Migration Institute has grown less than 10 percent. Magdalena Carral, the commissioner of the institute, says she has about $140 million a year to spend on security at all of Mexico's airports, seaports and land borders. The United States spends some $700 million to secure its borders, and the United States Border patrol has tripled in size. Ms. Carral said that while more than 14,000 American agents patrol the 2,000-mile United States-Mexico border, Mexico has fewer than 329 agents covering its 700-mile border with Guatemala.

"The question we must ask is are we in a better position today to stop the flow of migrants through the country, and the answer to that question is yes," said Mr. Creel, the interior minister. "But if you ask do we have systems that are able to stop anyone and everyone from crossing, the answer is no.

"We do not have such systems," he added, "but neither does the United States."

That point became alarmingly clear two weeks ago with the arrest of a 48-year-old woman carrying a ripped South African passport and $7,000 in assorted currencies.

United States Border Patrol officers detained the woman, Farida Goolam Mahomed Ahmed, at an airport in the border city of McAllen, Tex. Initial reports following Ms. Ahmed's detention indicated that she was being held in federal custody as a suspected terrorist. Two weeks later, authorities have failed to charge Ms. Ahmed with anything more than illegally entering the United States and altering a passport.

Still, Mexicans and Americans said her case was a reminder that the border they share remains porous and unsafe, that migrant smuggling thrives and that the threat of a terrorist entering from Mexico remains real.

One Mexico City columnist proclaimed this week, without supporting evidence, that Al Qaeda was operating in Mexico. The American authorities and a high-level intelligence official in Mexico's Interior Ministry dismissed the column. However, the Mexican official acknowledged that the possibility is worrisome.

"Welcome," he said, "to my nightmare."

Gustavo Mohar, a migration expert who formerly served in Mexico's Foreign Ministry, said: "It is a very frightening scenario, but real. If we continue to have a border that allows tens of thousands of people to cross without papers, you never know when you are going have someone cross who is a threat."