"For shame!" shouted about 200 demonstrators gathered at the court of Ancona in central Italy, accusing the justice system of misogyny and a "witch hunt", according to video footage posted by Italian media.
The reasons behind the 2017 acquittal were only made public on Friday when the country's highest court of appeal ordered a retrial, according to the Cronoche Ancona Daily.
The men, both Peruvians, had initially been convicted in 2016 of raping a Peruvian woman in 2015, but the Ancona appeals court threw out the charges, saying the woman's story was not believable.
The woman had claimed one of the men raped her while the other stood guard, after they had spiked her drink with drugs.
Doctors said her injuries were consistent with rape, and found traces of a date rape drug in her blood.
But the judges ruled it was "not possible to exclude the possibility that it was" the alleged victim who "organised the 'exhuberent' evening," according to the reports.
They said the man accused of rape "didn't even like the girl, to the point of having stored her number in his phone under the nickname 'Viking', an allusion to an anything but feminine figure, rather a masculine one".
"The photograph present in her file would appear to confirm this," they added.
The case will be reheard from scratch by a court in Perugia at at date yet to be determined.
