Italians are voting Sunday in a referendum on constitutional reform that could gauge anti-European Union sentiment in the country.
Polls opened at 7 a.m. local time (0600 UTC) and are set to close 16 hours later (2200 UTC).
About 51 million Italians are eligible to vote, and full results are expected in the early hours of Monday.
Voters will decide whether to approve a proposal by pro-EU Prime Minister Matteo Renzi to increase his powers so he can push through economic reforms that the government has said would bring political stability in Italy.
However, almost all opinion polls over the past two months have suggested Renzi will be defeated.
He has promised to resign if he loses the ballot.