Around 50 million Italians are eligible to vote in the two-day election, the country's first for five years.
Both candidates used their final election rallies to appeal to undecided voters.
Political analysts say as many as a quarter of Italians have yet to make up their minds and the result could go either way.
The 69-year-old media magnate, Mr Berlusconi arrived at his rally in Naples on the back of a motorcycle driven by an aide.
He said Italy's family values and "Christian identity" were under threat and likened his opponent to Stalin and Pol Pot.
Former European Commission president Mr Prodi appealed for national unity and said Italians needed change.
The last opinion polls before a pre-election ban put Mr Prodi ahead with a five percentage point lead over the Prime Minister.