Rome: Italians began voting on Sunday in local elections that are the biggest test for Prime Minister Matteo Renzi since his centre-left Democratic Party (PD) triumphed in a vote for the European parliament a year ago.

Some 22 million voters will elect new governments in seven of Italy’s 20 regions and around 1,000 municipalities following a campaign dominated by corruption allegations against one of Renzi’s most prominent candidates.

The 40-year-old premier needs a convincing result to maintain momentum for labour, education and constitutional reforms which have met fierce resistance from trade unions, the political opposition and the left wing of his own PD.

The centre-left currently holds power in five of the seven regions. If Renzi improves on that tally against a divided opposition he will continue to look unassailable some 15 months after he come to office.

Opinion polls, which have a poor track record in Italy, suggest the PD will remain easily the biggest party. However, internal party tensions erupted on Friday when parliament’s anti-mafia committee released the names of 16 local election candidates suspected of corruption or organised crime links.

By far the most high-profile of the “impresentabili” or “unpresentable” candidates was Vincenzo De Luca, Renzi’s candidate for president of Campania, a crime-ridden southern region around the city of Naples.

De Luca, a powerful local party baron, is accused of graft and also has a conviction for abuse of office which may prevent him from taking his seat even if elected.

He denies wrongdoing and threatened to sue the anti-mafia committee’s chairwoman, Rosy Bindi, for putting him on the list.

Renzi accused Bindi, who comes from the left wing of the PD, of using the committee “to settle scores inside the party”.

Opposition parties are hoping to capitalise on the furore.

The anti-establishment 5-Star Movement, which polls say is the second-largest party, hopes it can emulate the success of Spain’s anti-austerity Podemos party in local elections last weekend.

The anti-immigrant, anti-euro Northern League expects to gain from the unprecedented numbers of African and Middle-Eastern migrants that have arrived on Italy’s shores this year. More than 5,000 have been rescued in the Mediterranean this weekend alone and are being brought to southern Italian ports.

The PD should win comfortably in its strongholds of Tuscany, Umbria and Marche in central Italy as well as in Puglia in the south-east, while the Northern League is likely to keep power in the north-eastern region of Veneto.

That means the main battle grounds are likely to be in north-western Liguria, where the PD vote may be weakened by a breakaway left-wing candidate, and in Campania, where De Luca is in a tight race with the centre-right incumbent.

By noon (1000 GMT) around 16 per cent of the electorate had voted. That was up five points compared with the corresponding election in 2010, but on that occasion voting took place over two days. No exit polls will be released when polls close at 11pm and results will trickle in through the night.