long characterized the law as a tailor-made measure aimed at shielding Mr. Berlusconi from the four criminal trials he is currently facing.On Monday, the prime minister acknowledged the defeat."On each theme, Italians have made their position clear. The government and Parliament will now have to take into account this result," Mr. Berlusconi's office said in a statement.Monday's outcome is notable not just for the lopsided vote but also because it comes just weeks after Mr. Berlusconi's conservative coalition was badly defeated in local elections. Though the premier still has the majority in Parliament he needs to govern, his popularity has been falling in recent months.Italy's center-left opposition parties, which had widely campaigned for people to vote, were jubilant. Opposition leader Pierluigi Bersani hailed the result as a crucial sign of the need for political change and called for Mr. Berlusconi's government to resign."This referendum marks a divorce between the Italians and the government. At this point, the government has to leave and lead to new elections," Mr. Bersani told a news conference.Conservative government officials tried to play down the political meaning of the vote. "The fact that the referenda reached the quorum doesn't change anything for the government," said Ignazio La Russa, Italy's Defense Minister and a strong Berlusconi supporter.Italy's chronically feeble economy, however, is weighing heavily on young people in particular, and many here are fed up with the premier's legal woes, including most recently his trial on charges of paying for sex with an underage woman and abusing his power to cover it upcharges the premier denies.The Northern League, Mr. Berlusconi's party's most important ally in Parliament, also is getting fed up. "Two weeks ago, we got a first slap in the face in local elections. Now the referendum has dealt us a second slap in the face," said Roberto Calderoli, Italy's minister for legislative simplification and a key Northern League official.Italy abandoned nuclear energy in 1987shortly after the Chernobyl nuclear accidentby voting against it in a referendum similar to Monday's. In the current vote, the Fukushima Daiichi nuclear crisis in Japan drew people to the polls. As in other European countries, Italy earlier this year imposed a moratorium on its nuclear plans, but Mr. Berlusconi's government was hoping to resurrect them longer term by building several plants across the country. Early on Monday, as the results were coming in, Mr. Berlusconi said that without the possibility of nuclear plants, Italy would have to "strongly commit" to renewable energy.Many others pushed for a nuclear revival in the country. "Italy will spend a lot of money for energy needs" without nuclear energy, said Chicco Testa, head of Italy's Nuclear Forum and former chairman of Italian utility Enel SpA. Mr. Testa was one of the main backers of the 1987 referendum against nuclear power, but has since changed his stance: "There is plenty of gas out there and coal, but I don't know what the prices will be in 10 years' time as they are tied to oil. Based on past experience, I see higher oil prices."The two water-related referenda asked Italians