Prime Minister Matteo Renzi, who called the referendum, said he will resign if it's a 'no' vote
Exit polls suggest the Italian prime minister has lost a referendum on reforming the country's constitution.
Matteo Renzi has promised to resign if he failed to get his proposals through.
Polls for national broadcaster Rai and the La7 television channel both projected the "No" camp as winners by a margin of at least 54% to 46%.
However, commentators have noted that Italian exit polls have previously proven to be unreliable, so a definitive picture will not emerge until early results are announced overnight.
The public today voted 'yes' or 'no' to constitutional change which would reduce the power of the Italian senate in order to streamline Italy's notoriously gridlocked legislative process.
Prime Minister Renzi, who called the referendum and wants the change to be voted for, has staked his political career on winning.
If the vote is 'no', he has pledged to resign; a promise he made when the polls suggested he'd win by a margin.
Prime Minister Renzi's promise to go has, for many voters, turned the poll into a referendum on his leadership.
Prominent populist opposition parties have pushed for a 'no' vote to force Mr Renzi from office.
On the far right of politics and powerful in the north of the country, the Lega Nord, or Northern League, is led by Matteo Salvini.
An MEP in the European Parliament, Mr Salvini's popularity as the party leader has been boosted by his anti-immigration views and pledges to federalise the country.
The Five Star Movement is more of a threat to the centrist government led by Mr Renzi.
Led by comedian Beppe Grillo, it is populist, anti-establishment and now counts the powerful mayors of Rome and Turin among its number.