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The election, which concludes on Monday afternoon, is being followed closely by investors; their memories are still fresh of the potentially catastrophic debt crisis that saw Mario Monti, an economics professor and former bureaucrat, summoned to serve as prime minister in place of Silvio Berlusconi 15 months ago.
A weak Italian government could, many fear, prompt a new dip in confidence in the European Union's single currency.
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 the painful austerity measures imposed by Monti's government and deep anger over a never-ending series of corruption scandals. Berlusconi's centre-right has also revived.
“I'm not confident that the government that emerges from the election will be able to solve any of our problems,” said Attilio Bianchetti, a builder in Milan, who voted for the anti-establishment 5-Star Movement of comic and blogger Beppe Grillo.
The 64-year-old Grillo, heavily backed by a frustrated generation of young Italians hit by record unemployment, 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 8 am (0700 GMT) with polling booths remaining open until 10 pm on Sunday. They will open open again between 7 am and 3 pm on Monday. Official results are expected by early Tuesday.
A group of women's rights demonstrators greeted Berlusconi when he voted in Milan. They bared their breasts in protest at Berlusconi, who is on trial for having sex with an underage prostitute.
