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 president Mauricio Macri, and 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.

"Judges: Never again. No free genocidists," read banners in the Plaza de Mayo of Buenos Aires.