Saddam' Hussein's execution was denounced virtually across Italy's political spectrum.
Silvio Berlusconi, the conservative opposition leader and former president who sent thousands of Italian troops to Iraq, calling the killing a political error.
Saddam's execution for crimes against humanity also prompting a hunger strike from Marco Pannella, leader of Italy's Radical Party, who thanked the mayor from his hospital bed for lighting up the Colosseum.
Italy is also spearheading a campaign for a United Nations moratorium on the death penalty.
"The execution of Saddam Hussein has stirred a debate," said Michele Lembo, a demonstrator outside the Colosseum.
"We ask people to think about what happened and propose an alternative."
Italy and all other European Union countries have outlawed capital punishment.