The former economics professor was flanked by leaders of key parties in his center-left Union coalition, including Piero Fassino of the Democrats of the Left and Franceso Rutelli, Margherita party.
Rutelli said earlier: "Next Monday will be the day that Silvio Berlusconi returns to the opposition and Romano Prodi will become head of government."
Prodi, a former prime minister as well as the European Commission's ex-president, was leading in the opinion polls by up to five percentage points before voter surveys were suspended two weeks ago under electoral rules.
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