Rome mayor Gianni Alemanno, a right-wing politician and sponsor of the event, described this gesture as a "clear sign of the profound pain" his city was feeling, and called on the two marines to be returned to Italy.
An image of the two marines (Massimiliano Latorre and Salvatore Girone) and of the Italian flag was projected on the monument, one of Italy's most iconic buildings, with a sizeable crowd watching the event.
The mayor's attempt was to keep up the issue alive and whip up emotions as the diplomatic dispute between Italy and India refuses to settle over the trial of the marines.
The marines are accused of killing two Indian fishermen off the coast of Kerala in February 2012.
Italy contends the incident happened in international waters and that its own courts have jurisdiction over the case.
The Supreme Court in January ruled against Italy and ordered the central government to set up a special fast-track court in New Delhi to handle the case. That has not happened yet.
Next hearing of the case is on April 16, when the Supreme Court will review the matter of the special court and whether the NIA will probe the case.