"Julia" is a short, plump woman in her 60s who has worked all her life as a clerk in Buenos Aires, Argentina. About 2 1/2 years ago her daughter-in-law called her at 4 in the morning to report that her son had been taken away by what appeared to be police. "Julia" says she has been unable to find any trace of her son ever since.

Yesterday she and "Maria," another "Mother of the Plaza de Mayo," went to Capitol Hill under the auspices of Amnesty International to gain attention for what they say are 15,000 men, women and children who have simply "disappeared" since the military coup in Argentina in March 1976.

To dramatize their cause, they presented white cotton scarves embroidered with the names of different "disappeared" to women leaders, including Patricia M. Derian, the assistant secretary of State for human rights, who said she represented not only the State Department but First Lady Rosalynn Carter and the senior women in the White House.

The occasion was emotional; the women talked about babies as young as 20 days old who have been swept up in a military dragnet originally aimed at rounding up terrorists who had brutalized the country before the coup. They are merely mothers, they said, not political players - and all they want to know is what has happened to their children.

According to Amnesty International, which won the Nobel Prize in 1977 for its work in helping political prisoners around the world, most of the 15,000 "disappeared" are not terrorists. Some are leftists, some are labor union leaders, some are intellectuals - such as the 31-year-old head of the Buenos Aires Psychologists Association.

The mothers - who numbered, at their peak, 2,000 - are the most recent in an apparently unending flow of people who see themselves as human rights victims seeking solace in the United States. Speaking graphically of torture, punishment for opposing government policy, random arrests and imprisonments, house arrests, intimidation and other harassment, they've come speaking for "prisoners of conscience" in the Soviet Union, Chile, Paraguay, Brazil, South Africa, El Salvador and other countries.

But Amnesty spokesmen say the situation in Argentina is different because so many have not received even a travesty of due process; they simply are picked up, some to return as corpses, minus their hands to prevent identification, floating in the river. A few are released, but most are believed to be in "clandestine" detention camps.

"Julia" and "Maria" were afraid to be photographed; they would not give their real names and insisted that factual details about their lives be changed.They are afraid that they, like some other protesting mothers, will "disappear."

A spokesman for the Argentine Embassy here, Massini Ezcurra, said he questioned figures cited by Amnesty International. He did say that between November 1974 and June 1979, 8,713 people were arrested in Argentina in connection with suspected terrorist activities; of these, 5,913 have been released and 1,723 are still under "executive-power arrests." That means the president decides they should be arrested and jailed indefinitely without trial. Two weeks ago, Ezcurra said, the government said they would hold trials for those people.

The U.S. government apparently agrees more with Amnesty's figures, as Congress voted to cancel the sale of arms and to vote against loans to Argentina from the Inter-American Development Bank.

"It's in our national interest to stand for the principles that we believe in," said Derian. "Originally the military junta was faced with the very real problem or terrorism. In the process [of eradicating them], they dehumanized the enemy: they became more dangerous than the terrorists themselves. The root cause was panic, inexperience, and a failure to be deeply rooted in the civilizing aspects of constitutional rights and due process."

"We ask only for justice," said "Maria," in heavily accented English. "Try them - if they are guilty, punish them. But if they are not guilty, release them."

Both women are apologetic about their English. "Maria" speaks more then "Julia"; both have been traveling for about a month in Europe and are now here to plead for international pressure on Argentina. They met in the Plaza de Mayo - Argentina's equivalent of Lafayette Square - where mothers gathered every Thursday in a vigil of demand.

"Maria" is a teacher; she, like many of the "disappeared," is Jewish. She says her efforts to get information about her son and daughter through legal channels have resulted in "nothing." Now in her 50s, she is a widow. She and "Julie" were selected, along with two others who have since returned to Argentina, by the mothers to make this trip. It is the third time the group has sent representatives to America.

Yesterday as they presented the embroidered scarves to 20 women such as Rep. Barbara Mikulski (D-Md.), Rep. Patricia Schroeder (D-Col.) and columnist Mary McGrory, the two kissed each woman as she took the scarf.

"Maria gave away one scarf with the names of her own children embroidered on it; but she could not reveal this for fear of later being identified.

The mothers and their sponsors chose to make their public ceremony to women. "We think maybe women can have a better understanding of the problem," said "Maria." " . . .they say that time will cure all injuries. But no mother will ever forget her children."

According to "Maria":

Last january, 44 mothers were stopped by police as they arrived for their weekly protest and taken to a jail cell. Many of them were pushed into a jail cell in which lay the dead body of a young man who was the victim of a car crash. They were told he could be one of their sons.

They were told not to meet again in the Plaza de Mayo. If they did, police warned, they could end up in jail permanently.

Babies born to women who were pregnant at the time of their arrest have been given away or sold, they claim. One woman, Estrella Iglesias Espasandin, was released on May 15 after a year and nine months of imprisonment. She is a Spanish citizen who has joined her husband in France, but she gave a report of her experience, including torture:

"The torture consisted of applications of electric current to the genitals, breasts, toenails, mouth and gums; stretching my arms; mostly my right arm until they dislocated it; putting rats on my face and between my legs," she wrote . . . "During the entire trip (to jail) they socked me in the face and on my body. They also told me that I wasn't detained, nor had I desappeared, nothing like that; now I'd just been absorbed, kidnapped, 'sucked up.'"

She also gave a list of names of 18 people who have been "disappeared," people who were still alive at the time she was released.

Ezcurra siad there is no torture in Argentina. "How could I agree with that?" he said. He said the two mothers would not be punished for speaking here, "unless they are involved in some terrorist-related activity."

"I've had people tell me "intellectual subversion is as dangerous as real subversion,'" said Jeannie Harris, wife of the foreign service officer who until recently was assigned to spend full time on human rights at the embassy in Argentina. F. Allen Harris has not yet been replaced.

Harris, who was praised lavishly by the two mothers, had his three-year human rights assignment cut short recently to join the SALT task force in the State Department. Even withing the embassy in Buenos Aires, the commitment to deal with this problem is less than enthusiastic, according to several observers. "It's too unpleasant to document," one source said.

"Maria," dressed in a bright red blouse and blue skirt, said she at first could not believe that her son had simply disappeared.

"How can you believe a thing like that? It's incredible. He had no weapons, he was not political. The most he might ever have done is write a slogan on a wall. There are many of them people who may have sympathy with the left, idealistic. But they are what they call 'parsley' - how do you say, insignificant ."

Her voice is a little hoarse from talking so much; the phrases are starting to sound practiced. Her eyes are red. "I'm exhausted," she said. CAPTION: Picture, Argentine women present scarves to Reps. Patricia Schroeder and Millicent Fenwick, by James K.W. Atherton - The Washington Post