Each weekday morning, a long line of Argentines snake along the high metal fence surrounding the U.S. embassy here. Interspersed among visa seekers and businessmen are half-a-dozen mothers, fathers or spouses waiting to tell the United States about jailed or missing family members.

In Uruguay, a U.S. embassy official makes regular calls to the government to check on the status of a Brazilian journalist imprisoned in the Montevideo jail. Although there is no ostensible U.S. involvement in the case, a relative said, no other foreign representation there has taken much of an interest.

When a Chilean labor leader, banished to a remote corner of the country, defied authorities and flew into Santiago a few weeks ago, the U.S. embassy there sent an official to the airport to be seen by the Chilean government as an "eyewitness to the arbitrariness" of the subsequent arrest.

Existing international agreements prohibit the United States from officially petitioning on behalf of foreign nationals in their own countries. Yet in a striking reversal of long-standing policy. U.S. embassies in a number of Latin America's military-ruled nations have taken on visible, if largely unofficial, human rights roles on a local level.

Many embassies spend much of their time, a U.S. official in one humar rights "trouble spot" said, "looking for unofficial ways" to keep steady rights pressure on the host governments.

Those unofficial pressures include constant references to U.S. human rights policy in ostensibly unrelated official conversations, receiving and at least listening to anyone who shows up at the embassy with a human rights problem, and the highly visible official U.S. presence at local human rights-oriented-functions.

David Popper, when he was ambassador to Chile, made a point of attending anniversary ceremonies at the Catholic Church's human rights organization. According to one U.S. source, Popper's attendence last year made him conspicuously late for an official reception given by Foreign Minister Patroco Carvajal.

In such areas as Argentina, where local rights organizations are largely uncoordinated and fearful of reprisals, he embassies have become principal human rights nerve centers.

Several Latin governments have been accused of rights violations ranging from mass kidnapings and detentions to torture and deprivation of political and property rights. In those countries, U.S. embassies maintain their own lists of missing persons.

All maintain a network of local contacts on human rights matters through church, professional, political and trade union organiztions that often operate undergroound.

The high level of involvement under the Carter administrationis is, in all but a few cases like Popper's, an abrupt turnabout from previous U.S. diplomatic policy. In the past, the embassies in Latin America served primarily as miltary and poltical intellgence stations, or as accounting offices for the distribution of US aid.

Many State Department employees in the area still find the role of human rights watchdog both uncomfortable and improper. Occasionally such policing causes conflicts of interest as when a Foreign Service bureaucrat sees a military or economic assistance program he administers cut because of negative human rights reports his embassy has sent to Washington.

But what began early this year as, in some cases, a grudgingly administered policy, recently has emerged as a primary activity in embassies in Argentina, Uruguay, Chile, El Salvador and, to a lesser degree, in Brazil, Paraguay and Nicaragua.

Embassy officials long skeptical of human rights pressure now tend to keep their opinions to themselves, while closet rights advocates under previous administrations have emerged as open activists.

Those lame-duck ambassadors who opposed Presidnet Carter's rights emphasis, including Robert Hillin Argentina, who frequently disagreed on record with Carter's world "policeman" attitude, and Ernest V. Siracusa, who was accused in the U.S. Congress of soft-pedaling reports of violationsin Uruguay, have been removed.

In some cases, the new ambassadors are long-time human rights advocates, such as Robert White in Paraguay. Even former Arizona Gov. Raul Castro, whose appointment to the embassy in Buenos Aires was criticized by U.S. human rights groups, has mentioned the rights issue in virtually every public - and reportedly every private - meeting with Argentine officials since his arrival last month.

While many in Washington still find Carter's human rights policy vague and ill-defined, officials in most embassies here, according to a diplomat in Chile, "have reached a consensus on the dimension" of human rights' importance in U.S.-Latin relations.

One reason why rights activity in these embassies is more extensive than in other parts of the world is apparently the willingness of the Latin governments to accept U.S. diplomatic pressure.

While "there is no question that they don't like the raised eyebrows," the diplomat in Chile said, the Chilean government has become convinced that "the dynamic of U.S. relations with Chile is human rights."

Often, government response to proests is minimal.

"But the government here deserves some credit," said a U.S. official in Argentina. "They have opened a dialogue on human rights."

While an informal inquiry on a disappeared person more often than not comes back stamped "no record," many believe that most people who disappear in Argentinal are in the custody of the police or military. Every request is answered.

"The informal request [on Argentine nationals] are accepted by the grace of the Argentine government," the official said. "As a policy, the government has been responsive."

The Latin situation conrasts with that in the Soviet Union, where American diplomatic contacts with Soviet dissidents are carefully controlled, and made only with approval from high embassy levels.

The extent of human rights activity in an individual Latin embassy depends on the extent of the problem, the local resources available in terms of legal and church assistance, the aggressiveness of exbassy personnel, and the level of interest in Washington.

In Argentina, as in other embassies, official U.S. inquiries can be made only for the half dozen Americans held for various crimes. Numberous held for various crimes. Numerous on behalf of any disappeared Argentine or other foreign national with relatives or friends on the Unites States.

U.S. congressmen have requested information on more than 100 Argentines who have disappeared, and presure from U.S. labor, religious and political groups often gives the embassy an excuse to ask about some of the estimated 10,000 to 20,000 cases of disappeared persons.

The embassy saves its heaviest pressure for certain celebrated cased that it uses as "examples." Among those are the cases of imprisoned Argentine publisher Jacobo Timerman, or disappeared biochemist Adolfo Moldavsky. The Timerman and Moldavsky families, along with others, met with State Department human rights coordinator Patricia Derian during her visit here last month with Secretary of State Cyrus Vance.

Vance himself met with a number of local human rights and religious leaders, meetings that would be highly unlikely during an official visit to Moscow.

The families in hundreds of less publicized cases turn up at the embassy regularly.

While their cases connot be directly appealed, U.S. legislation requiring the State Department to submit regular reports to Congress on the human rights status of U.S. military and economic aid recipents enables the embassy legitimately to keep track of them.