Italians cast their ballots in more than 1,300 municipal councils on Sunday, with the main focus on the big cities of Bologna, Milan, Naples, Turin, and Rome.

A series of corruption scandals in Rome has hurt Renzi's center-left Democratic Party (PD), whose candidate Roberto Giachetti was trailing M5S's Virginia Raggi by more than five percent in opinion polls ahead of Sunday's vote.

Dozens of former city officials and business leaders have been on trial since last November in a case known as "Mafia Capitale." They are accused of corruption and other behavior that led to millions of euros being removed from the administration.

If no candidate gets more than 50 percent, as polls suggest, then run-off elections will be held on June 19.

A breakthrough for the Five Star movement, a eurosceptic populist grouping that has vowed to fight graft and provide services, would be a major blow to Renzi. The vote comes four months ahead of a referendum on constitutional reforms and then national elections in 2018.

A Five Star victory in the capital would provide the movement with a platform to pursue its policies and the momentum its leaders believe will turn it into Italy's main opposition party.

Another key city for Renzi is Milan, where DP's Giuseppe Sala could face a challenge from a moderate centre-right rival.

Polls close at 11 p.m. local time (21.00 UTC) on Sunday night.

cw/jm (AFP, dpa, Reuters)