Prime Minister Silvio Berlusconi has vowed Italy would keep troops in Iraq despite yesterday's bombing that killed 18 Italians.
The suicide attack looked likely to stoke anti-war sentiments in Italy, where some opposition parties demanded its 2,300 troops be withdrawn. But Mr Berlusconi, a staunch ally of President Bush, said Rome should stand firm.
"No intimidation will budge us from our willingness to help that country rise up again and rebuild itself with self-government, security and freedom," he told the Senate in an address broadcast nationwide after the attack in Nassiriya.
Italian Defence Minister Antonio Martino said the Italian dead were 12 Carabinieri military police, four soldiers and two civilians - Italy's highest military death toll since World War II.
Mr Berlusconi urged Italy and its leaders to remain united.
"If there ever was a day when controversy should be silenced and when all Italian citizens should show solidarity with those who have taken on the lofty task of defending the values of our democracy, then this is the day," he said.
Flags were lowered to half-staff across the country and many cities declared today a day of mourning. It was the first fatal attack on Italian forces since they were sent to Iraq in June to help stabilize the country.
Pope John Paul, who opposed the Iraq war, expressed his "most firm condemnation" for what he called a "vile attack" against a mission of peace. Mr Bush sent condolences, saying he "appreciated the sacrifices".