Leaders from the Italian opposition participated in the protest despite rainy conditions on Saturday.
Organisers estimated a turnout of 70,000; the police did not provide a figure.
"The war is not yet over and we must continue to keep mobilised," Oliviero Diliberto, secretary-general of the Italian Communist party, said.
Green party president Alfonso Pecoraro Scanio said: "100,000 dead is a good reason to demand the withdrawal of Italian troops from Iraq," a reference to the huge number of civilians killed both during last year's war and since the fall of Saddam Hussein.
"It's not enough to sign unitary motions on the withdrawal. We must also mobilise, as thousands of pacifists have done today. This large turnout shows that peace is an issue in Italian public opinion that can mobilise people."
Rome has approximately 3000 soldiers in Iraq - the third largest US-allied contingent in the country, after the US and Britain, which have about 140,000 and 8500 troops there respectively.