Prosecutors have charged the 74-year-old premier of paying for sex with Moroccan belly dancer Karima El Mahroug -- stage name Ruby Rubacuori -- when she was only 17.
Iaia Caputo, a leader of the protests around the country, told Britain's Sky News, "The Ruby case has revealed a system of political selection based on an exchange of sex and power."
Even prostitutes and nuns were expected to join the rallies to show their disgust with Italy's billionaire leader, who is accused of hosting sex parties at his mansion.
Opposition MP Sandro Gozi told Sky News, "In any other democratic country in the world, the prime minister who is accused of such shameful and serious allegations would have already resigned. But Italy is not a normal country yet, unfortunately."
Recent polls, however, indicate that Berlusconi would win a snap election.
A judge is expected to rule Monday or Tuesday on prosecutors' request to send the case to trial. If approved, it could start by April.