Thousands of protestors have marched across Rome massed behind a large banner reading No War - No Bush.
Riot police with shields and batons scuffled with small groups of masked protestors, some of whom threw bottles and flares, but there were no reports of serious injuries.
Police say that 7,000 people took part in Rome's major demonstration against US President George W Bush's 36-hour visit, while the organisers claimed 100,000 had marched.
Earlier, the President held talks with Pope John Paul at the start of a three-day visit to Europe to attend World War II ceremonies.
The Pope pressured Mr Bush over what he called recent deplorable events in Iraq and described the situation in the Middle East as one of grave unrest.
The Pontiff also called for a speedy return of sovereignty to Iraq.
'It is the evident desire of everyone that this situation now be normalised as quickly as possible with the active participation of the international community,' the Pope said.
The meeting was one of Mr Bush's first as he began his Europe visit to celebrate the 60th anniversary of D-Day and the liberation of Rome.
Meanwhile, Mr Bush has nominated his Sudan peace envoy, John Danforth, as the new US ambassador to the United Nations.
If confirmed by the US Senate, Mr Danforth would take the post left vacant by John Negroponte, who recently became US ambassador to Iraq.
Bush to meet Berlusconi
Tomorrow, Mr Bush is due to meet the Italian Prime Minister, Silvio Berlusconi, one of his strongest allies in the Iraq war.
Speaking ahead of the visit, Mr Berlusconi said he was worried about possible violent protests and called on Italians to receive Mr Bush as the leader of a great nation.
Tight security surrounds the trip amid fears of a terrorist strike.
Earlier today, the US President met his Italian counterpart Carlo Azeglio Ciampi at the Quirinale Palace.
