The demonstrations on Wednesday took place in various parts of the Italian capital as political leaders attended the annual Republic Day military parade under tight security.
The demonstrators, who have dubbed themselves The Disobedient, held their own march nearby but police were on hand to keep them away from the parade route.
There were some minor scuffles with police but no immediate reports of serious incidents.
The protesters, including some leftist parliamentarians, chanted "Peace Now!", "Troops Out of Iraq!", "Bush Go Home," and "Italy hates war".
This year's parade, in which military, police and volunteer units marched as planes flew overhead, has taken on added controversy because of the war in Iraq.
Most Italians opposed the US-led invasion of Iraq last year and there have been many calls for Rome to withdraw its 2700 troops, the third-biggest contingent to US-led occupation forces in Iraq after the United States and Britain.
Message to Bush
Demonstrators unfurled rainbow coloured peace flags from many of Rome's ancient bridges at the same time and one pulled a black hood over the statue of an angel to protest against the prisoner-abuse scandal in Iraq.
The Republic Day parade was
marked by minor scuffles
One banner read "Liberate Rome from Bush". US President George Bush is coming on Friday to mark the 60th anniversary of the liberation of Nazi-occupied Rome by allied forces.
Another banner showed some of the pictures at the centre of the prisoner-abuse scandal in Iraq which has inflamed the Arab world and embarrassed the United States by revealing the ugly side of its occupation in Iraq.
"This is the democracy of Bush, Blair and Berlusconi," it said.
Security officials consider the parade, which was attended by President Carlo Azeglio Ciampi and Prime Minister Silvio Berlusconi, a dress rehearsal for the Bush visit when about 10,000 uniformed police will be on hand.
"Our demonstration is
to say that there also exists an Italy with a
flag of peace, a flag
with a rainbow against war, against militarism and against a military parade that shouldn't
be taking place"
"Our demonstration is to say that there also exists an Italy with a flag of peace, a flag with a rainbow against war, against militarism and against a military parade that shouldn't be taking place," said Paolo Cento, a member of the Green Party.
Some officials in Rome fear a repeat of the violent clashes that marred a G8 summit in the northern city of Genoa three years ago when one protester was killed by police.
Opposition to Italy's participation in Iraq grew after 19 Italian troops where killed there in November and a private security guard was killed in April.