From their prison 2,400 miles away in Mexico City, a group of Americans, most of them "mules" in the parlance of the drug world, have been lobbying the august Senate Foreign Relations Committe for help.

A bizarre lobby, yes, but these are difficult times for the 575 or so Americans, many of whom have been brutalised or extorted by guards, in Mexican prisons. Their hopes were lifted when the United States signed an unprecedented treaty in November that would allow them to finish their terms in U.S. prisons, but after months of delay, their patience is strained.

And so on May 3, a committee of prisoners asked the U.S. consul general in Mexico City for help in arranging telephone calls to members of the Foreign Relations Committee, which must act on the treaty.

Sixteen days later, Raphael Diaz Wilson of New York was on the telephone, collect, to Sen. Jacob K. Javits (R.N.Y.). In a barely audible voice and with references to "diet problems" and other hardships, he asked, "Do you intend to vote favorably on it?"

"I am favorably disposed to it," Javits told the 31-year-old Wilson, who has served five years of a 13 1/2-year term. But Javits also cautioned him that there were constitutional issued to be resolved: Can the United States imprison someone convited in another land?

"I support the intent of the treaty," Sen. Charles H. Percy (R-III) told another prisoner in a cabled response to questions laid out by the prisoners' committee and forwarded through the State Department.

But he, too, noted "difficult constitutional issues."

The efforts of the 60 men at Santa Marta, according to a U.S. official in Mexico, have been followed by a request from 40 women in a nearby prison fro telephone talk with President Carter.

"Yes," says Consul General Vernon McAninch when asked if there are unsavory people involved. "They did something wrong, they should pay some penalty. Their hopes are up and their patience is wearing thin. They really believe they belong at home . . .

"You really have to visit here and be on the scene to appreciate the problem."

Of the 575 imprisoned Americans, about 80 per cent are being held on drug charges, and most of those are "mules," McAninch said in a telephone interview. "Mules" are people arrested with hard drugs, a few grams on up to several pounds' worth.

McAnich said there is at least one convicted murderer, and State Department officials in Washington said other cases involve drug quantities ranging from a couple of seeds to "tons and tons of stuff" - including that of the crew of an airplane that crashed while taking off, it was so heavy with drugs.

For many Americans, arrest and trial in Mexico has led to beatings bordering on torture, forced confessions, extortion and overcrowded and sordid conditions in some prisons. Last September, some prisoners went on a hunger strike to protest what they saw as U.S. laxity on a Mexican proposal for an exchange of prisoners.

In November, such a treaty was signed. It would allow U.S. citizens to choose to finish their terms in U.S. jails, and Mexican citizens here to return home to conclude their sentences. About half of the 1,200 Mexican prisoners in the United States would be eligible; the rest are imprisoned on immigration charges, which disqualify them.

For some Americans, transfer to the United States would mean early freedom since many, like Raphael Wilson, would be eligible for parole. U.S. prisoners are first considered for parole after serving one-third of their sentence; Mexico has no parole.

Detlev Vagts, a Harvard law professor who drafted the treaty while serving for a year as the State Department's counselor in international law, said the major U.S. constitutional question is whether someone can be kept in custody here "without ever having had a trial that met U.S. standards."

But he said that the fact that transfers would be voluntary should overcome that objection.

The Foreign Relations Committee is to conduct hearings Wednesday and Thursday to discuss such problems and to take testimony from parents with children in Mexican prisons.