Prime Minister Silvio Berlusconi is facing the prospect of fresh humiliation today following local election defeats, after Italians flocked to the polls to vote in opposition-backed referendums.

Although the government had urged its supporters to stay away, turnout in the vote yesterday and today was above the 50 percent required for a move to scrap Italy's plans to return to nuclear power to have legal force.

Another referendum targeted a law passed by Berlusconi's government last year that has allowed the legally-embattled prime minister to stay out of court, in a sign of simmering discontent with the Italian leader.

The voting is expected to go against Berlusconi since opposition supporters came out in force to vote after country-wide mobilisation.

"An enormous mass has gone out to vote, including people who voted centre-right and those who voted centre-left," Antonio di Pietro, leader of the opposition Italy of Values party, said after the turnout data were released.

The vote comes shortly after Berlusconi's Italy of Values party suffered a humiliating defeat in mayoral elections in Milan and Naples last month.

There were signs too of cracks within the ruling coalition, with Interior Minister Roberto Maroni criticising the government over the economy, a major source of unease among many Italians at a time of near-zero growth.

Umberto Bossi, the leader of Berlusconi's junior but highly influential coalition partner, the Northern League, said: "Berlusconi has lost the ability to communicate on television. That's the simple truth."

Commentators said the success of the referendums would force a fundamental rethink within the ruling coalition.

"The referendums are communicating an alarming message for the entire centre-right," centrist daily Corriere della Sera said in an editorial.

"If what the polls are revealing is a loss of contact with the country, then the problem concerns the entire alliance," it said.

Business daily Il Sole 24 Ore said this was the "most arduous" challenge for Berlusconi since his election victory in 2008.

But the government warned against making too much of the referendum results.

"If the left really respects voters they should not... manipulate this in order to give it a meaning or a political effect that it does not have," Daniele Capezzone, a spokesman for the People of Freedom party.

The nuclear vote aimed at putting a definitive stop to Berlusconi's aim of restarting Italy's atomic energy programme by 2014, plans that are already under a temporary moratorium following the Fukushima disaster in Japan.

"Following a decision being taken by the Italian people, Italy will probably have to say goodbye to the issue of nuclear power stations," Berlusconi said at a joint press conference with Israeli counterpart Benjamin Netanyahu.

"We will have to commit strongly to the renewable energy sector," he said as polls closed, even though the result of the vote had not yet been confirmed.

Italy abandoned atomic energy with a referendum in 1987 after the Chernobyl disaster, but Berlusconi has made its reintroduction a major policy goal.

His government argues that it would slash electricity bills, reduce Italy's energy dependency and be better for the environment.

Opinion polls however suggest that nuclear power remains deeply unpopular.

Daniel Cohn-Bendit, leader of the Greens in the European Parliament, has said that a vote against nuclear power in Italy "could open a serious phase of reflection in other member states" of the European Union.

A vote against the "legitimate impediment" law would also be a strong signal of voter disenchantment over the prime minister's legal woes.

The prime minister, 74, is a defendant in three trials involving allegations of bribery, fraud and paying for sex with a 17-year-old girl.

A Constitutional Court ruling has already curbed much of Berlusconi's legal protection but the "legitimate impediment" law being voted on at the weekend is still officially in place.

"If we manage to meet the quorum on legitimate impediment it will show that people have turned against the Berlusconi system," Margherita Lodoli, 28, a charity worker in Rome, told AFP after casting her ballots.