MONTEVIDEO, URUGUAY -- Opponents of a law providing amnesty for military men accused of human rights atrocities have announced collection of more than half a million signatures, enough to challenge the law in a referendum.

The signatures, if authenticated, would represent one-quarter of the voters in this nation, which is struggling to maintain its democracy almost three years after the end of military rule.

In the early 1970s, Uruguay's military and police were locked in a fierce battle to repress Marxist urban guerrillas known as the Tupamaros, who had sought to overthrow what was then a venerable democracy. The military formally seized power in 1973, although by that time the guerrillas had been contained.

The repression of the guerrillas carried over, however, to targeting of members of the socialist and communist parties, to organized labor and others on the left. International human rights groups say that as many as 50,000 people were jailed, and many tortured, during the 12 years of military rule. At one time this seaside nation of 3 million had the highest number of political prisoners per capita in the world.

These accounts indicate that about 170 people were kidnaped by the security forces here, in some cases in cooperation with the military across the Plate River in Argentina. All are presumed dead.

Backers of the referendum to repeal the amnesty have pasted up thousands of posters to remind Montevideo's 1.5 million citizens of nine children who are among the missing. The posters show a highchair knocked to the ground, a teddy bear alongside.

"The kidnaped kids are depending on you," reads the poster. "Sign now." The poster added impetus to the petition drive for a referendum on the military amnesty law, said Ines Previtali, secretary of the group pressing the issue.

"When democracy returned, people expected justice to be done," she said. "We owe it to the kids who are still missing. We cannot pretend that nothing has happened here." The anti-amnesty campaign began after President Julio Sanguinetti, who received the tacit support of the outgoing military regime in his 1984 presidential campaign, steered through Congress the bill exempting military men from trial.

Sanguinetti said the law was necessary to avoid a constitutional crisis and a showdown between the military and the courts.

Last week, the referendum campaigners said that nearly 555,000 signatures had been collected. According to Uruguayan law, at least 25 percent of the voters -- or 553,000 people -- need to sign the petitions before a referendum can be called. Spokesmen say they hope to gather 80,000 more signatures, as some could be invalidated by an electoral court. The official canvass may take six months.

The commission's apparent success in the first round of the challenge to the amnesty clearly has rattled the military, which is already preoccupied by questions of pay and promotions.

Politicians who once played down the importance of the referendum now appear to be taking it seriously. Many compare Uruguay's situation with that of Argentina, where President Raul Alfonsin's government was forced to back down on military prosecutions after officers revolted in early April.

The referendum proponents "are operating on pure emotion," said one veteran political columnist, who asked not to be identified by name. "They still haven't explained what will happen if they win. We'd be back to where we started -- with the military saying they won't accept being prosecuted, and the government not having the means to force them to. That can't be good for democracy."

Sanguinetti recently expressed confidence that the law will stand. "The measure that brought this chapter of our nation's life to a close will undoubtedly be supported by the majority of the people," he said.

The repression in Uruguay never reached the levels of that in Argentina, where thousands were secretly killed. Sanguinetti's critics point out, however, that not a single Uruguayan military or police officer has had to testify before a court. Alfonsin's government, by contrast, set up a blue-ribbon commission to investigate the illegal repression and more than a dozen former military leaders were convicted and jailed.

"The amnesty law has not resolved any problems," said referendum committee chairwoman Matilde de Gutierrez Ruiz, the widow of a former president of the chamber of deputies who was kidnaped and murdered in Buenos Aires by Uruguayan officers in 1976.

Last week, the newspaper Con Todos published a purported military memorandum of a meeting Sanguinetti held with 218 Army officers in May. The document, whose authenticity was not denied by the government, said he took credit for keeping officers from being extradited to Argentina to testify in the murders of Hector Gutierrez Ruiz and former senator Zelmar Michelini.

An incident Sept. 9 showed how the military and many civilians often seem to speak virtually different languages when dealing with rights issues, past and present. An Army doctor,. Nelson Marabotto entered a local bank and was recognized by an employe who had been political prisoner at the notorious Punta de Rieles prison. People in the bank jeered Marabotto and shouted antimilitary slogans.

The vice director of a military hospital, Marabotto had been expelled by the local doctors' union as a result of accusations that he improperly treated women political prisoners.

Some newspapers said there were shouts of "assassin, torturer!" Others said Marabotto was spat upon, a charge later denied by bank employes. The doctor was escorted out by a police guard.

A recent study by the American Association for the Advancement of Science, "Uruguay's Military Physicians: Cogs in a System of State Terror," said doctors played a significant role in the repression.

The study said it found "sufficient evidence to support the conclusion that physicians' complicity in torture was systematic and widespread." It complained, however, that its investigations were hampered in part "in December 1986 when . . . parliament approved an amnesty bill pushed by President Julio Sanguinetti."