The Mexican government reportedly has information that Chilean and Argentine undercover agents may be planning terrorist actions against leading members of the large South American political exile community here.

According to well-placed government sources, Mexico's Interior Ministry has ordered its secret service to keep close tabs on at least four Chilean military intelligence operatives and 16 Argentine secret police agents who are known to be living here.

The Chilean and Argentine agents have been conducting surveillance of the homes of leading leftist exiles and photographing exiled gathering points, the sources said. They said an increasing number of exiles have also complained of receiving threatening telephone calls.

The agents' presence has created much concern in the large exile commuity here, which includes nearly 3,500 Chileans, Argentines and Uruguayans who have fled rightist repression at home over the last three years.

Chilean exiles in particular fear further attempts to eliminate members of Chile's former Socialist government following the 1974 assassination of former Vice President and army commander Gen. Carlos Prats and his wife in Argentina, and the murder of former Foreign Minister Orlando Letelier, who died when his car was blown up in Washington last September.

Although other prominent Chileans have received threats in New York, Rome, Paris and several Latin American capitals, exiles in Mexico until recently have felt safe from rightist terrorist agents.

But last year, Clodomiro Almeyda, another former foreign minister under Allende and a key figure in the exiles' political reorganization, was urged to leave Mexico "for a safer place." Soon afterward, Almeyda moved to East Germany. The warning had come from Letelier who, just weeks before his own assassination, informed him that he, Almeyda, ranked high on a list of people to be eliminated.

Mexican authorities are now providing protection for the Chileans who may be targets of assassination plots, including Allende's widow, Hortensia, his former ambassador to Mexico, Hugo Vigorena, and his former economics minister, Pedro Vuskovic.

A number of persons in the 2,000-member Chilean exile community reportedly have received threats by letter and telephone, and security has been increased in the Casa de Chile, their community center.

Although government sources would not disclose the identity of the four alleged Chilean military intelligence operatives, a Mexican press report this week described two Childean military delegates to the International Civil Aviation Organization here and one university professor as agents of DINA, Chile's national intelligence directorate.

The fourth agent was only mentioned by the initial "C".

In the Argentine exile community - totaling almost 600 teachers, scientists, union leaders and artists who fled rightist repression, first under Isabel Peron and later under the military government that took over last March in Argentina - nerves have been set on edge by information that 16 Argentine police have arrived here since last September by way of Italy.

In recent weeks, Argentines say, several persons have been spotted taking photographs outside the Casa de Aregentina," a small, inocuous-looking building that is a gathering point for Argentina," a small, inocuous-looking City.

In Anzures, a neighborhood where many Argentine exiles live, exiles claim that "suspicious-looking men" have been seen surveying their homes for hours on end, particularly that of Ricardo Obregon Cano, former governor of Cordoba Province.

One Argentine, also from Cordoba, reported last week that he was deeply shocked when he saw three members of the Cordoba military police near his front door. He told a close friend that he recognized them immediately as men who had arrested his son in Cordoba. After his son died from torture in Cordoba jails, the Argentine fled to Mexico with his family.

A report in the Mexican newspaper El Sol, which included physical descriptions of some of the alleged Argentine policemen, has prompted the Argentine ambassador here to protest to the Mexican government, calling the report "a provocation and an exaggeration."

Rudolfo Puiggros, former rector of Buenos Aires University and a community leader here, said: "I wish it were an exaggeration. The reality is far worse." Puiggros said that he and numerous others have received phone calls from people with Argentine accents who say "We'll kill all you Argentines here, as well as your Mexican friends."

Although no incident of violence has been reported so far, and events have been limited to intimidations, the developments here are all the more worrying to the exiles because Mexico is effectively the only country in Latin America that still maintains an open-door policy for exiles.

In the past, countries such as Venezuela, Costa Rice, Ecuador and Panama have received persecuted leftist or independent critics of the rightist military governments that now dominate most of Latin America.

But exiles here say those countries have become more and more reticent in receiving them. Moreover, in Chile, Argentina, and Uruguay, many foreign embassies and counsulates are watched to prevent people from seeking asylum. Three years ago, Mexican President Luis Echeverria launched this continent's largest effort to receive, house and provide jobs for people escaping the southern part of Latin America.

When the government here changed last December, there was much apprehension among the exiles that the new administration would be less tolerant of their presence. But the government of President Jose Lopez Portillo has repeatedly assured them there would be no change.

At present, the Mexican embassy in Buenos Aires houses Argentina's former Peronist president, Hector Campora and his son, who, for the last nine months, have refused an exit visa.

In Mexico City, the exiles' fear has been eased somewhat by government's promises to police the foreign agents. But exiles say they no longer feel as safe in Mexico as they used to. While they are aware that their safety would be virtually guaranteed in Communist countries, many who are not Communists are reluctant to go there.

In addition, a large number of people who arrived in Mexico as tourists, rather than under the laws of asylum, are practically stuck. The Argentine and Uruguayan embassies here reportedly refuse to issue or renew passports to the exiles and have said that they will only provide documentation for people returning home.

There is no Chilean embassy here since Mexico broke relations in 1974, and some Chileans say that it is useless to turn to the Brazilian embassy, which handles Chile's affairs.