Each day at dawn, soldiers and immigration officials fan out across the mountain roads of this isolated region along the southern border, stalking bus depots and markets, pulling their vehicles through jungle mud, asking in each village for strangers.

Their job, like that of their American colleagues in the U.S. Border Patrol hundreds of miles to the north, is to catch illegal aliens. After decades of turning a blind eye toward the hundreds of thousands of Mexicans pouring illegally into the United States, Mexico is now suffering from an invasion of its own.

Hundreds of families, single young men, aging peasants and woman carrying children cross into Mexico every day from the south, feeling violence and poverty in Guatemala and El Salvador. Those without the resources and stamina to head farther north, hitch-hiking or riding buses to Tijuana and into the United States, remain here, begging for food and shelter, looking for work.

There are no exact figures on the wave of refugees. The office of the United Nations High Commissioner for Refugees in Mexico City, which offers funds for food, legal and medical aid to refugees, estimates that Mexico harbors at least 70,000 Salvadorans. The figure for Guatemalans may be similar, which movement back and forth across the border varying almost like a barometer of Guatemalan political violence.

In the parish house next to a church outside Tzimol, a priest talked to a woman and her small from Santa Ana in El Salvador who had been walking for the past two months, crossing 300 miles of Guatemala to reach Mexico. She said her husband and two other sons had been killed by the Salvadoran police "and I don't want to lose my last son."

Five other adult refugees, including two women carrying babies, were Indians from near Nebaj in Quiche Province in Guatemala. "They've told me they are not political," the priest said. "The guerrillas come to their village, then the Army comes. Then the soldiers shoot the people even if they have nothing to do with it. When the war is over, they say, they want to go back."

Mexico has always prided itself on its generosity to the politically persecuted and currently harbors exiles from 13 Latin American nations. But in this country where unemployment is high and poverty runs deep, the unprecedented flood of neighbors is clearing creating a serious dilemma for the government.

Mexico's official response to the influx has been alternating tolerance and crackdown, which the aliens department of the Interior Ministry was unwilling to detail. "Our policy is to be humanitarian rather than legalistic in applying the law," said one official. But he conceded that so far this year Mexico has deported four times as many people as it had in the same period last year. Social workers here say that a refugee commission set up by the Interior Ministry has so far failed to provide the large-scale help for which it was created.

There has been strong criticism in the Mexican press of alleged imprisonment, rough treatment and extortion of undocumented refugees by Mexican officials. Several recent articles have directly pointed to Mexico's own frequent complaints about mistreatment of its undocumented citizens in the United States.

Church and social workers in contact with refugees say that while official policy may be lenient, many refugees tell stories about the network of pirates they encounter on their way. These range from paid smugglers and guides that get them across the border, to immigration officials and soldiers who detain them and demand bribes to let them go.

While the Mexican peasantry here on the high plains above the border appears to have little understanding of the violence in Guatemala and El Salvador, its traditional suspicion and fear of its own Army and police have led many peasant families to offer temporary food and shelter to the refugees, according to local priests.

Military officials at the Army post outside Comitan declined to discuss the refugees. But a local immgration official, who asked not to be named, explained official policy.

"The government does not want refugees accumulating along the border in settlements or camps. That only leads to trouble. We don't mind off families, but if we find groups we have no choice but to deport them."

The official emphatically denied reports that Guatemala's leftist guerrillas used the Mexican border zone as a sanctuary. "The guerrillas don't come here.But if we had refugee camps they might come, or at least people would say they were here."

Two months ago, the official said, the Army discovered about 500 Guatemalans living in the jungle. "I went along when they were picked up," he said. "Their situation was dramatic, families with 10, 12 children, living under a sheet of plastic stuck on poles. They lived off fruits and roots, they told us, but they looked like they hadn't eaten for days. And that jungle is dangerous, there are snakes the size of a telephone pole."

When the Army started loading them on trucks, the people panicked, the official said. "They said we are not asking for anything, just let us stay here. They'll kill us on the other side."

Samuel Ruiz, the bishop of San Cristobal has apealed from the pulpit that refugees not be handed over the Guatemalan authorities. He said in an interview that the church had no means to follow up the fate of the people deported. Yet he said he had reason to believe that some of those handed over to the Guatemalan authorities have disappeared.

Mexican officials have responded that they have no way of telling which are economic or political refugees.

The dividing line between Mexico and Guatemala is only one-fourth as long as the 2,000-mile border with the United States. But it seems even more porous. Flanked by vast plantations on the Pacific plain, it climbs, unmarked, up into the Sierra Madre and disappears in the vast virgin forest that divides North from Central America.

Two rivers, the Suchiate and the Usumacinta, serve as part of the dividing line. But the unpatrolled rivers are easily crossed by refugees in small boats or on rafts or lage inflated tractor tires used by local entrepreneurs to take refugees across for a few Guatemalan quetzales. A quetzel is worth $1.

"There's no way we can do this job," said one border agent who was manning a roadblock."Most people don't come by road and nobody can patrol the jungle. There's just a few of us, and we have no helicopters or motor boats. The whole is impossible."

Then with a smile he added: "I guess that's the way it is for the Americans up north."