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.