Italians are voting in a referendum on constitutional reform that will decide the political future of Prime Minister Matteo Renzi, who has promised to resign if voters fail to back his reform plans.
About 51 million Italians are eligible to vote on Renzi's plan to drastically reduce the role of the upper house Senate and reduce powers of provincial governments.
Turnout was very high by Italian standards, with 57.24 percent of voters having cast their ballots by 18:00 GMT, according to the Interior Ministry.
Nearly two thirds of the electorate had voted in some parts of prosperous northern Italy but the turnout was much lower in the south - a pattern which was seen as a potential boost to the premier's survival hopes.
Italians living abroad have voted by post over the past days and the ballots flown to a hangar on the outskirts of Rome.
If the reform moves forward, the Senate would lose much of its power. Instead of the current 315 senators, there would be 100, and rather than being directly elected, they would be selected by regional assemblies.
The reform would dissolve Italy's 110 provinces, Italy's second-level administrative divisions - considered expensive and redundant - while municipalities, metropolitan cities and regions would remain.
The populist Five Star Movement, the biggest political rival to Renzi's Democratic party, has been backing the No campaign in the referendum.
The Five Star Movement has also sought to capitalise on Renzi's declining popularity, a sluggish economy and the problems caused by tens of thousands of migrants arriving in Italy.
"God willing it's over. A new era starts tomorrow I hope," said Matteo Salvini, leader of the far-right Northern League, after voting in Milan.
With all the opposition parties lined up against the reform, a victory for Renzi would be a surprise and represent an enormous personal triumph for Italy's youngest prime minister who often appeared to be fighting the campaign single-handed.
Renzi, 41, voted mid-morning in his Tuscan home town of Pontassieve, but made no comment.
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In the final days of frenetic campaigning, he insisted that the public mood was changing, focusing his attention on the millions of Italians who said they were undecided.
Turnout, expected to be 50-60 percent, could be crucial. Pollsters say lower participation could favour Renzi, as hostility to his reform is strongest among young voters and those in the poor south, segments of the population that often don't bother to vote. The referendum does not require a quorum to be valid.
A turnout above 60 percent could also make the result more unpredictable as it would suggest many voters who said they planned to abstain ended up going to the polls.
Financial markets and Europe's politicians fear victory for the opposition "No" camp could cause political instability and renewed turmoil for Italy's battered banks, pushing the eurozone towards a fresh crisis.
Economy Minister Pier Carlo Padoan sought to calm nervous markets on Friday, saying there was "no risk of a financial earthquake" if 'No' wins, though there may be "48 hours of turbulence".
Source: News Agencies