Talks have taken place at the Vatican between Pope Benedict and the US President, George W Bush.
It is thought the discussions centred on differences over Iraq and the controversial transport by the CIA of alleged terror suspects to interrogation centres in Europe - so-called extraordinary rendition.
10,000 police have been deployed in central Rome ahead of expected large scale protests.
Mr Bush had said before the meeting that his intention was mainly to listen to the Pope.
Earlier, he met Italy's President, Giorgio Napolitano, a former communist.
His visit coincides with the opening a of the trial in an Italian court of 25 CIA officers and a senior US Air Force officer who are accused of abducting a Muslim cleric in Italy in 2003.
Later, the US President thanked the Italian Prime Minister Romano Prodi for his support in Afghanistan and Lebanon, and said the two countries shared common values.
In a joint news conference in Rome Mr Bush also called on Syria to stop disrupting the Lebanese government of Prime Minister Fouad Siniora.
Asked about his meeting earlier today with Pope Benedict, Mr Bush said he the pontiff was a smart and loving man, and said they had discussed the situation in Iraq.
Later on, demonstrators protesting against the visit threw bottles at police and overturned tables in central Rome. Police fired tear gas to try to control them.
