Since her two children disappeared three months ago, apparently following arrest, Alicia Valdes has been knocking on prison doors, waiting in police stations, pleading with guards at Mexico City military camp. But she has found neither her daughter Rosalina, 21, nor her son Eduardo, 23.

Laborer Antonia Garcia has made the same rounds. His daughter Hortensia, 20, disappeared on June 7, after being picked up by police, according to an anonymous caller.

"What police? What prison? What cell? She must be somewhere," Garcia despairingly said to a human rights group last week. "God only help me if she's dead."

As reports of repression and police and army brutality in Mexico have increased over the last 20 months, the fate of these and another 373 people who have disappeared has provoked bitter criticism and started to taint Mexico's liberal image abroad.

Although the disappearance of political activists in other Latin American nations has been well-publicized, similar occurences in Mexico have received little attention perhaps because figures of international repute are rarely involved.

Recently, however, increasing repression here has led to a renewed interest in Mexico by international human rights organizations. Although Mexico has publicly welcomed President Carter's human rights campaign, U.S. officials for the first time raised the issue with the Mexican government earlier this year.

Mexico's authorities have traditionally used violence against opposition groups. But the hard-handed crackdowns, ranging from the use of the army to break strikes to reported kidnapings and torture by police, have provoked loud protests by human rights groups whose records show that 376 persons have "disappeared" in the last four years, many after arrests devoid of such niceties as formal charges, warrants and similar procedual protections.

This month, after a long campaign by action committee and opposition groups, the government responded by passing a new amnesty law. In recent weeks, more than 30 of about 400 persons jailed on political charges have been freed. Several had been held without trial for five years. More persons will be released, the authorities say, "once the paperwork is done."

Before the new amnesty law was passed, officials insisted there were no political prisoners, also denying the frequently heard charges of torture and of "missing persons" held in clandestine jails or inaccesible inflitary quarters.

Privately, however, one high government official recently conceded in an interview that he regarded the use of torture during interrogation as a "necessary evil." And Rosario Ibarra, whose 21-year-old son has been missing for two years, said she was informed recently by a high official that the government would do its best to present the missing persons in public "as soon as possible."

Ibarra said she was told that the effort is being hampered by conflicting opinions within the government about what should be done and by the fact that the missing have been taken into custody by a variety of police agencies.

A majority of the missing are either student or peasant activists or leftist guerillas who are often picked up by the army or by the police's dreaded "White Brigade." This special anti-guerilla unit was formed last year, by people drawn from Mexico's principal security police forces and the military police.

Although Mexico saw considerable urban and rural guerrilla activity in the early 1970s, most extremists have been grabbed by Mexico's efficient security apparatus and only two small groups are still sporadically active.

Therefore, defense lawyers say, many people arrested or missing are involved only in nonviolent political work or are simply extremists' relatives who are held for questioning or moral pressure for indefinite periods.

The list of the missing, for example, includes Armando Tecia Parra, 14, who was picked up in 1976 because his two sisters were thought to belong to a leftist extremist group, and Reyes Mayoral, 65, who was pulled out of his house Aug. 28, 1977, by plainclothes police who told him his son was a dangerous guerrilla. Neither has been heard of since.

Defense lawyers Guillermo Andrade and Carlos Fernandez, both of whom have had long experience defending political detainees, say there is ample evidence, based on testimony from people later released, that the government holds people in clandestine prisons or in the dreaded military camp No. 1, which is reputed to be Mexico's main "torture camp."

Such disappearances staged by army or police may not be official policy, but the two-year-old government of President Jose Lopez Portillo on the whole has cracked down hard on th peasants' and workers' groups that were encouraged to organize during the government of his predecessor, Luis Echeverria.

This administration has sought to rebuild rightist confidence in the government, not only by adopting conservative economic policies but also by trying to suffocate unrest among peasants and the labor sectors that have been hit hardest by the current economic crisis. The high inflation of the last few years has diminished the purchasing power of the poor dramatically, while unemployment stands officially at a record 51 percent.

It is hard to get accurate reports about rumored repressive army actions in the countryside although some well-documented cases of army abuse include the arrest of peasant leaders and the raping of women and girls.

But in the cities, crackdowns against labor have been highly public, influence of "independent" trade unions that are increasingly rebelling against the government-controlled workers' federation.

In at least half a dozen cases this year, soldiers and police have been used to break strikes and arrest labor leaders. One such incident, which caused outrage in the press, was the army's raid on the copper mine La Caridad in the northern town of Nacozari. Soldiers arrested many of the striking miners and construction workers, whose leaders said they were tortured.

In Mexico City this summer, thugs - reportedly paid by the government - beat up people attending a union meeting at the General Hospital. When a general protest strike followed, more than 100 plainclothes police and soldiers raided the hospital, beat up and arrested nearly a hundred strikers and finally charged union leaders with theft.

These events illustrate the paradoxes of Mexico political life, so bewildering to outsiders. While with one hand the government is tightening its control over dissenters, with the other it is making political concessions to bring the opposition into the political mainstream. The government this year legalized three new parties, one rightist and two leftist, including the Communist Party.

But although such promises of political participation are causing some artisfaction among the government's opponents, friends and relatives of prisoners and missing persons say they are not satisfied.

Last week, the National Committee, for the Defense of Political Prisoners, which has broad nationwide support, announced its second hunger strike this year to put pressure on the government. It demands that the government "abide by its own amnesty law, and above all, once and for all, present the 376 disappeared persons publicly or explain their deaths."