While Diego Cammarata looked set to win a second term as Palermo mayor in Sicilian local elections on Monday, rival candidate Leoluca Orlando demanded the race be annulled because of alleged fraud.
Early projections gave Cammarata, a member of opposition chief Silvio Berlusconi's Forza Italia party, a 13% lead over Orlando.
But Orlando's press office said the centre-left candidate had called Interior Minister Giuliano Amato to ask that the vote be voided.
"Given the extent of the fraud that is emerging, we won't be commenting on the election outcome," the press office added.
Supporters of Orlando, a former Palermo mayor and ex-regional government chief, alleged that numerous ballots had been wrongly assigned to Cammarata.
They said that in one polling station, they had found 200 ballots that had been completed by the same person using the same mark and a pencil that had not been supplied by voting staff.
Police also said that at least seven people had been reported for photographing their completed ballot slips on their mobile phones.
They noted that vote buying was suspected in such cases.
Centre-left MPs wrote to the interior ministry to complain in the run-up to the Palermo vote that they had evidence of parties backing Cammarata handing out free mobile phones, money, food vouchers and boxes of pasta to voters in poor neighbourhoods.
Voting for 156 new mayors and a new provincial government in Ragusa in Sicily, a centre-right stronghold, began on Sunday and ended at 15:00 local time on Monday.
Turnout was unusually high for the island at 72.43% with a total of 2.2 million Sicilians entitled to vote.
The most important race was in Palermo but other big mayoral contests were in Agrigento and Trapani.
Projections put the centre-right opposition ahead of the centre left in most of the contests.
The opposition immediately said it had inflicted a "crushing defeat" on the centre left.
Former premier Berlusconi, who was narrowly defeated by Premier Romano Prodi in last year's general election, said that "this vote is an unequivocal message intimating the end of Prodi's government".
But Prodi said the elections were "not a national test".
Other members of the governing centre-left coalition said the opposition's "triumphalism" was "ridiculous".
