BUENOS AIRES — Worried that the men who committed some of Argentina’s most heinous human rights abuses could be freed from jail years early, hundreds of thousands of protesters took to the country’s streets on Wednesday.
The demonstrations were in response to a Supreme Court ruling this month that reduced the sentence of a man convicted of crimes against humanity during the country’s military dictatorship from 1976 to 1983.
The court’s decision led to a flood of requests for the same leniency from others imprisoned for kidnapping, torture and murder. Activists warned that the ruling could pave the way for the early release of some of the era’s most notorious offenders.
But widespread repudiation of the ruling by the public, culminating in the day of protest, which organizers said had drawn half a million people in Buenos Aires alone, forced an uncharacteristically quick and united response from political leaders here.
After just two days of debate, Argentina’s Congress on Wednesday passed a bill intended to prevent any shortening of prison terms for those sentenced for crimes against humanity. The vote was nearly unanimous, almost unheard-of in the country’s divided political landscape.
Hours before the protests began, President Mauricio Macri also came out against the decision, saying he was “against any tool that favors impunity.” His administration had initially emphasized the need to respect the independence of the courts, but changed tack as opposition mounted, and helped push the bill through Congress. It became law on Friday.
Experts say the developments of the week follow a historical pattern.
“On human rights issues, the political and institutional actions have always come after a demand from society as a whole,” said Gastón Chillier, the executive director of the Center for Legal and Social Studies, a human rights group in Argentina.
While it now appears very unlikely that hundreds of human rights abusers will be leaving prison ahead of schedule, activists said the court’s decision exemplified the way in which Mr. Macri had adopted a less aggressive approach to dealing with one of the darkest chapters in Argentina’s history.
Two of the three justices who signed on to the decision were appointed by Mr. Macri, who took office in 2015, and activists say the ruling fits a pattern of his administration’s tamping down efforts to seek justice for the atrocities carried out during the dictatorship.
Trying those crimes was a centerpiece of the policy of the former presidents Cristina Fernández de Kirchner, who served from 2007 to 2015, and Néstor Kirchner, her husband, who led the country from 2003 to 2007.
The court decision “surprised and angered us, but it isn’t incoherent with the philosophy of the current government,” said Estela Barnes de Carlotto, who leads Grandmothers of the Plaza de Mayo, a group of women searching for the hundreds of newborn babies stolen from their mothers in clandestine detention centers during the dictatorship.
“It took Macri a whole week to say anything,” said Victoria Durán, a demonstrator at the Wednesday protest in Buenos Aires.
As protesters packed the Plaza de Mayo square in Buenos Aires, they sang: “What happened to the Nazis will happen to you. Wherever you go, we will go after you.”
Marta Mamaní, 64, went to the march carrying a photo of her sister Olga Mamaní and her brother-in-law Luis Torres, both killed by the military after they disappeared in June 1976.
“The people who killed my sister can’t be walking the streets,” she said. “This march is a huge victory, but it tastes very bittersweet.”
The man to be freed early by the court’s ruling, Luis Muiña, was convicted of torture and other crimes.
When Gladis Cuervo, 77, heard that the top court had reduced Mr. Muiña’s sentence, “I immediately felt despair, anxiety and a lot of pain,” she said.
Mr. Muiña was a security guard at the Posadas Hospital in Buenos Aires Province who helped set up a clandestine detention center there after the military junta’s takeover. Ms. Cuervo, a nurse at the hospital, testified at Mr. Muiña’s trial that he was one of those who tortured her when she was illegally detained for two months, breaking her sternum in beatings and burning parts of her body.
Mr. Muiña was sentenced in 2011 to 13 years in prison for kidnapping and torturing five people during the dictatorship.
In its 3-to-2 decision, the Supreme Court ruled that Mr. Muiña had the right to benefit from a law, which has since been revoked, that says the time a person spends in prison before a conviction should count double toward the sentencing total. A calculation by a lower court found that meant Mr. Muiña should have been freed in November last year. Mr. Muiña was already out of prison on parole when the ruling was issued.
The law, known as “two for one,” was in effect from 1994 to 2001 and had never before been used in a human rights case.
The attorney general’s office, an independent arm of the government now run by an ally of the previous administration, estimated that 278 people, or 52 percent of those sentenced for human rights crimes who are currently detained, could have seen a reduction in their sentence based on the Supreme Court ruling.
But the government is confident that none of these criminals will have their sentences slashed because of the bill passed by Congress clarifying that the “two for one” law cannot apply to human rights cases.
“We think it’s a law that will be applied immediately, so the ruling won’t have any real impact,” said Germán Garavano, the justice and human rights minister.
On Friday, the Supreme Court said it would gather all the pending appeals in its docket to apply the “two for one” law to crimes against humanity cases, opening the door to a new ruling on the issue as early as next month.
Until that happens, some of the most well-known names from that time of state-sanctioned terror will continue seeking shorter sentences.
The Rev. Christian von Wernich, a former police chaplain convicted in 2007 for his involvement in seven murders and 42 kidnappings, asked a judge to reconsider his life sentence. During his trial, witnesses recounted how Father von Wernich was present at torture sessions and used his position as a priest to extract confessions in what became an emblematic case that illustrated how the Catholic Church often cooperated with the military junta.
When discussing the Macri administration’s stance toward the time of the dictatorship, activists often cite how some officials, including the president, have questioned the number of people who died or disappeared during the era.
While official records estimate that 9,000 people were killed or disappeared, human rights groups say the real number is closer to 30,000. Mr. Macri told a BuzzFeed reporter last year that he had “no idea” what the number was.
“Whether they were 9,000 or 30,000,” he added, “it’s a pointless discussion.”
His government has dismantled human rights departments in several ministries that were helping investigate dictatorship-era crimes, and members of the administration have met with those who back the so-called two demons theory that the state actions were the result of a civil war against armed leftist groups.
That change in attitude has spilled over to the courts, activists say.
“Although the judiciary is relatively independent, it is clearly malleable to the agenda of the government that is in power,” Mr. Chillier, the human rights advocate, said.
Mrs. Kirchner commented on Twitter shortly after the decision. “Argentina has gone back 20 years in terms of human rights,” she wrote. “This ruling would not have happened in the previous government.”
While human rights groups celebrated the quick turn of events as a rare victory in the Macri era, they vowed to stay alert about what could happen next.
“This government has never been interested in human rights,” said Nora Cortiñas, 87, a leader of the Mothers of the Plaza de Mayo Founding Line, a group made up of women whose children were killed during the dictatorship. “We stood up to a dictatorship and are still fighting — why would we stop now?”