With few votes left to be counted in conservative Milan and battleground Naples, center-left candidates appeared headed for upset wins, ANSA reported.
"The results have gone beyond the rosiest predictions," the Democratic Party said in a statement issued from Rome.
The Italian news service said the margin of victory appeared higher than expected with center-left candidate Giuliano Pisapia holding a 55 percent-to-45 percent lead over incumbent Letizia Moratti in Milan's mayoral race and ex-prosecutor Luigi de Magistris ahead of Berlusconi's Naples mayoral candidate Gianni Lettieri by 65 percent to 45 percent.
Vote tallies indicate center-left candidates also held leads in other cities conducting elections, the first since Berlusconi was hit with several legal proceedings for alleged corruption, fraud and having sex with an underage prostitute.
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