ROME—Italians voted to abandon nuclear power for the foreseeable future, turning out in droves to cast ballots in a packet of referenda whose outcome is a sign of growing popular discontent toward Prime Minister Silvio Berlusconi's conservative government.

Mr. Berlusconi's administration had in past weeks urged people not to vote in the four referenda, which were organized by center-left opposition parties and which asked voters whether they wanted to overturn government laws...