The vote, called by centre-left Prime Minister Matteo Renzi, is formally on plans to streamline parliament but is expected to be used as a chance to register discontent.

Populist parties support a No vote.

The turnout has been very high by Italian standards - about 60% on average.
Nearly two-thirds of the electorate has voted in prosperous northern Italy but the turnout was much lower in the south.

Voting began on Sunday at 07:00 (06:00 GMT) and ended at 23:00 local time.
Matteo Renzi, who has said he will resign if he loses, is set to address the Italian people at midnight (23:00 GMT).