The first deaths of Italian troops in Iraq have sparked demands for the 3,000-strong force to be pulled out.
Berlusconi says Italy will not be deflected from its task
The government insisted the mission would continue, but opposition leaders said the UN should take over.
"The Italian servicemen must come back home. It is the only right thing to do now," said Pietro Folena from the Democrats of the Left.
"Italy should realign itself with France and Germany, withdraw its troops and start a debate with the USA and UK for them to stop their occupation.
"They should give full power and full mandate to the UN to deal with the difficult situation in that country," he added.
For the leader of Italian Communists, Oliviero Diliberto, the government carries responsibility for the Italian casualties in Iraq.
"In the face of this huge tragedy, I will ask only one question: in whose name have they been sent to death?" he said
"Had [the government] listened to us, our troops would not have been sent to die."
Green leader Alfonso Pecoraro Scanio said troops should be pulled out of Iraq.
"It is immoral to put the lives of thousands of young Italians at risk for Bush's pre-emptive war," he said.
"We hope that everybody will agree that we need to pull out our troops immediately."
Prime Minister Silvio Berlusconi, after consulting with Interior Minister Giuseppe Pisanu, voiced his grief over the losses, but insisted the operation should go on.
"No intimidation should distract us from our will to help that country rise up again and build up self-government, security and freedom," he said.
The European Commission president, Italy's Romano Prodi, on an official state visit to Senegal, has called for peacekeeping operations in Iraq to be taken over by the UN.
"We must move on to a phase where the UN has a greater involvement in achieving peace, a phase in which greater weight and power must be given to the Iraqi people within the Iraqi Government, Mr Prodi said.
"I had always thanked God because the Italians had been spared, but our fears were justified: this time it was our turn."
Pope John Paul II has sent a telegram of condolences to President Carlo Azeglio Ciampi, in which he expressed his "grief" over the "cowardly attack".
Sessions in both Italian houses of parliament have been suspended, and Defence Minister Antonio Martino was expected to address parliament on the attack at 1400 GMT.