Americans returning from Mexican prisons are likely to prove an embarrassment for the country that released them, a legal headache for the United States and a barrier to prisoner exchange treaties with other countries.

As more prisoners arrived today by plane from Mexico, Michael Abbell, the Justice Department attorney in charge of the transfer program, acknowledged that the U.S. government anticipates legal challenges by returning prisoners to the constitutionality of the treaty, which allows Americans convicted of violations of Mexican laws to serve their terms in the United States. Abbell said that such challenges, which probably would be based on the contention that the prisoners were denied due process in Mexico, could delay U.S. efforts to negotiate similar treaties with Canada and Bolivia.

But Abbell said he believes the Supreme Court ultimately would uphold the constitutionality of the treaty between the United States and Mexico.

"We're not in the business of legalized jailbreaks," he said. "We wouldn't have gone through with this if we didn't think it would stand up in court."

Those returning prisoners who were allowed to meet with reporters today said they did not plan legal challenges [WORD ILLEGIBLE] they are scheduled for early release, perhaps by Monday afternoon in some cases. American law provides for substantially less penalty for some drug offenses than does Mexican law.

But the prisoners painted a picture of systematic denial of rights in Mexico that is certain to be embarrassing to the Mexican government, which stiffened its drug laws and the enforcement of them primarily as a result of U.S. insistence. It also could lay the groundwork for the legal challenges anticipated from other long-term prisoners.

The prisoners repeated stories of torture, the widespread reporting of which over the last two years helped spark the prisoner repatriction treaty. They spoke of being jabbed with electric cattle prods at the time of arrest and of being threatened with physical abuse if they did not sign confessions. Some said they spoke no Spanish and signed the confessions without being able to read them.

But Don Bowen, 35, of Los Angeles, a spokesman for the male prisoners, expanded on previous charges by saying "the Mexican way of life is torture."

The prisoners were outspokenly critical of the U.S. Embassy in Mexico City, which they said did little to help them, and of conditions in Mexican prisons, which they said lacked elementary medical facilities and even aspirin. But they reserved their deepers bitterness for the Drug Enforcement Administration, which under the Nixon administration, launched a highly publicized "Operation Intercept" in a supposed effort to stop the heroin traffic from Mexico and South America.

As the prisoners told it, those who were arrested were not big drug smugglers but minor offenders involved with marijuana and cocaine, not heroin. Bowen said that the average amount of these drugs found on the Americans arrested in Mexico was a pound a person. An "inmate profile" prepared by the Bureau of Prisons showed that 96 per cent of those returning were convicted of drug offenses but that only 11 per cent of the cases involved heroin.

Most of the returning prisoners, expected to number 237 by Christmas, are white, young, middle-class and relatively well-off. By far the largest number, 41 per cent, are from California, which, together with Texas, New York and Arizona, accounted for the residences of two-thirds of those arrested.

Prison officials listed no returning prisoners from the District of Columbia or Virginia, and one, who was not identified, from Maryland.

There appeared to be substantial differences among the prisoners as to their actual involve with drugs. Some prisoners were arrested for possessing tiny amounts of marijuana. On the other hand, two California women, Elizabeth Lankton, 60, and Jeanne McMichael, 52, were picked up in Mexico City on charges of attempting to smuggle out seven pounds of cocaine in their suitcases.

McMichael, one of those who said she was tortured with a cattle prod after her arrest, said she survived in prison by painting and operating a breakfast-in-bed service. She said she made friends with long-term American prisoner, William John White, who visited her on the conjugal visits allowed in Mexican prisons.

White is scheduled for transfer to the United States next week, and McMichael said they will wed after they are paroled.

In a story typical of those told by many prisoners, McMichael said she had given an attorney $12,500 to free her, but had never seen him again. Some prisoners sold their homes and cars to obtain money for legal assistance or for sustenance in prison. Others, more fortunate, had substantial help from their families.

Bowen's mother, who organized a family group that pushed for the prisoner-exchange treaty, is a well-to-do real estate broker in the Los Angeles suburb of Hawthorne. She said she spent $75,000 to help her imprisoned son, including $30,000 for his support.

Prison officials described a tumultuous scene of happy, rollicking prisoners on the first plane home Friday night. The prisoners drank tomato juice, soft drinks and coffee and ate a dinner of ham and sweet potatoes. When the pilot announced that they had crossed the border into the United States, the prisoners cheered loudly for several minutes.

But such feelings went both ways. While all but 2 per cent of the 1,200 Mexicans imprisoned in the United States elected to stay here, those who returned seemed as happy as the Americans to get home.

One of the 36 prisoners who returned to Mexico Friday morning in a flight from San Diego told a photographer: "I would rather serve 10 years in Mexico than one more day in the United States."