The ruling, which would decrease jail time for those convicted of human rights abuses during Argentina's Dirty War, was later blocked by the country's Congress.
Tens of thousands of Argentines protested on Wednesday against a Supreme Court ruling that could decrease jail time for those convicted of human rights abuses during the country's 1976 - 1983 military dictatorship that killed as many as 30,000 people.
The ruling was widely criticised, including by the country's president, Mauricio Macri.
Argentina's Congress passed a law earlier on Wednesday to block future reductions of sentences for killings, torture, kidnappings and other human rights violations during the so-called Dirty War.
"I would like to congratulate the Congress for the speed at which it resolved the legal vacuum left by this unfortunate 2-for-1 law," Macri said in a press conference earlier on Wednesday.
"I am against any tool that is in favour of impunity, more so when this tool is applied to crimes against humanity."
TRT World's Kim Vinnell has more.
Two for one
The court said a law known locally as "two for one" that allows every day spent in jail before a final sentence to count for two days when more than two years have been served, could apply for human rights cases.
The Supreme Court's decision opened the doors to a thousand people who have been convicted and another thousand being held pending outcomes of trials asking for early release.
Many of the demonstrators raised white handkerchiefs symbolic of the Mothers and Grandmothers of the Plaza de Mayo, an organisation that highlights the confiscation of babies born to suspected dissidents during the dictatorship.
"We have shown, once again, that we do not want perpetrators of genocide, rapists and murderers to walk by our side," cried Taty Almeida, a veteran Argentine activist and key member of the Plaza de Mayo group.
Singers, actors, soccer players and politicians of all sides were among the crowd, which organisers said grew to some half a million people, in Buenos Aires.