Prodi was not hurt on Saturday when he opened a letter bomb sent to his home in the central Italian city of Bologna. But the incident raised questions about security in a city where an ultra-left urban guerrilla movement has been active in the past.
Investigators opened a probe into what they call an attempted "terrorist" attack and an attempt to overthrow democracy.
But Prodi played down the incident.
"Certainly it's not the best tidings for the next year," a smiling Prodi told journalists in Bologna. "Let's be on our guard, but above all let's not lose our calm."
The parcel bomb, which was planted inside a book and burst into flames when Prodi opened it, was sent less than a week after home-made bombs went off in rubbish bins in the street near his apartment, also without hurting anyone.
Prodi's police escort had been beefed up after the first bombs went off. Police suspect anarchists were behind the attacks, but local media said Prodi's escort was put under investigation after the parcel bomb got through them.
The country has been on high alert since a bomber killed 19 Italians last month in a strike against a military police headquarters in southern Iraq.
A newspaper on Saturday quoted Italian Prime Minister Silvio Berlusconi as saying he had received "precise and verified news of a (planned) attack on Rome on Christmas Day".
Berlusconi denied reports of a
possible attack on Vatican
The interview published in the right-leaning Libero daily said Berlusconi talked about "a hijacked plane above the Vatican" and "an attack from the sky".
The prime minister's office later released a statement denying he had given an interview to the journalist and had only exchanged Christmas greetings. The statement used a play on words to imply the journalist had invented the quotes.
But Libero stuck by its reporter and opposition lawmakers lambasted Berlusconi for treating the issue lightly.
"These are very delicate issues to be treated with reserve and responsibility," said Gavino Angius, the Senate leader for the opposition Democrats of the Left.
"We know there is a terrorist threat, but to talk about it like he did, saying one thing and then denying it... is sowing more fear than there already was."