Italians are voting in a referendum which is being closely watched for further signs of anti-establishment sentiment in Europe.
The vote, called by centre-left Prime Minister Matteo Renzi, is formally on plans to streamline parliament but is expected to be used by many as a chance to register discontent.
Populist parties have campaigned for a “No” vote. Mr Renzi has said he will resign if he loses.
Opinion polls suggest he faces defeat.
Voting began on Sunday at 07:00 (06:00 GMT) and ends at 23:00, with results expected early on Monday.
