Early results from the Italian general election are expected by tomorrow afternoon.
47 million Italians were eligible to vote in the election against a backdrop of the worst recession in 20 years.
There are concerns in Europe that an indecisive result could push up the country's borrowing costs and have a knock on effect on the single currency.
Italians began voting today in what is one of the most closely watched and unpredictable elections in years.
Opinion polls give the centre-left a narrow lead but the result has been thrown completely open by the prospect of a huge protest vote against painful austerity measures.
The cost-cutting began 15 months ago, when Mario Monti, an economics professor and former bureaucrat, was summoned to serve as prime minister in place of Silvio Berlusconi 15 months ago.
Mr Berlusconi's centre-right has recovered ground in public opinion, while the anti-establishment 5-Star Movement of comic and blogger Beppe Grillo has struck a chord with voters.
Mr Grillo, 64, is heavily backed by a frustrated generation of young Italians hit by record unemployment.
He has been one of the biggest features of the last stage of the campaign, packing rallies in town squares up and down Italy.
Italians started voting at 8am local time (7am Irish time).
Polling booths will remain open until 10pm tonight and open again between 7am and 3pm tomorrow.
Exit polls will come out soon after voting ends and official results are expected by early Tuesday.
Snow in northern regions is expected to last into tomorrow and could discourage some of the 47 million people eligible to vote in Italy to head out to polling stations.
The Interior Ministry has said it is fully prepared for bad weather.
Mr Monti and his wife cast their votes at a polling booth in a Milan school this morning, while centre-left leader Pier Luigi Bersani, the leader opinion polls suggest will have to form a new government, voted in his home town of Piacenza.
A small group of women's rights demonstrators greeted Mr Berlusconi when he voted in Milan.
They bared their breasts in protest at the conservative leader, who is on trial at present, accused of having sex with an underage prostitute.
Whichever government emerges from the election will have to tackle reforms needed to address problems that have given Italy one of the most sluggish economies in the developed world for the past two decades.
Final polls published two weeks ago showed centre-left leader Mr Bersani with a five-point lead, but analysts disagree about whether he will be able to form a stable majority that can make the economic reforms they believe Italy needs.
While the centre left is still expected to gain control of the lower house, thanks to rules that guarantee a strong majority to whichever party wins the most votes nationally, a much closer battle will be fought for the Senate, which any government also needs to control to be able to pass laws.
The eurozone's third-largest economy is stuck in deep recession, struggling under a public debt burden second only to Greece in the 17-member currency bloc and with a public weary of more than a year of austerity policies.
Mr Bersani is now thought to be just a few points ahead of media magnate Mr Berlusconi, the four-time prime minister who has promised tax refunds and staged a media blitz in an attempt to win back voters.
A strong fightback by Mr Berlusconi, who has promised to repay a widely hated housing tax, the IMU, imposed by Mr Monti last year, saw his support climb during a campaign that relentlessly attacked the "German-centric" austerity policies of the former European Union commissioner.
But the populist frustration Berlusconi's campaign tapped into has also benefitted Mr Grillo and many pollsters said his 5-Star Movement, made up of political novices, was challenging the centre-right for the position as second political force.