The story of the election in the eurozone’s third largest economy was shaping up to be the astonishing vote haul of comic-turned-political leader Beppe Grillo, whose 5 Star Movement has capitalized on a wave of voter disgust with the ruling political class.

Another surprise has been the return as a political force of billionaire media mogul Silvio Berlusconi, who was forced from the premiership at the end of 2011 by Italy’s debt crisis, and whose forces now had a strong chance of coming out on top in the Italian Senate. His main rival, the center-left Pier Luigi Bersani, appeared headed toward victory in Parliament’s lower house.

The unfolding murky result raised the possibility of new elections in the coming months and bodes badly for the nation’s efforts to pass the tough reforms it needs to snuff out its economic crisis and reassure jittery markets. In New York, the Dow Jones index plunged 200 points Monday on fears over Italian election gridlock.

While Italy’s postwar history has largely been one of revolving-door governments, it has never seen a hung parliament. Experts said that’s likely to change now.

‘‘This has never happened before,’’ said James Walston, a political science professor at American University of Rome. He predicted such a swirl of political chaos that new elections may need to be called as soon as this spring, when the new legislature chooses a new president who can be tapped to dissolve Parliament.

The Italian election has been one of the most fluid in the last two decades thanks to the emergence of Grillo’s 5 Star Movement, which has throbbed with anger at politics as usual. The movement came against a backdrop of harsh austerity measures imposed by technocrat Premier Mario Monti — who has fared miserably in the elections.

Many eligible voters didn’t cast ballots, and a low turnout is generally seen as penalizing established parties. The turnout, at under 75 percent — in a nation where it has historically been above 80 percent — was the lowest in national elections since the republic was formed after World War II.

Disgust with traditional party politics likely turned off voters, although snow and rain — this was Italy’s first winter time national vote — also could be a factor.

The decisions Italy’s government makes over the next several months promise to have a deep impact on whether Europe can decisively stem its financial crisis. As the eurozone’s third-largest economy, its problems can rattle market confidence in the whole bloc and analysts have worried it could fall back into old spending habits.

Bersani, a former communist, has reform credentials as the architect of a series of liberalization measures and has shown a willingness to join with Monti, if necessary. But he could be hamstrung by the more left-wing of his party.

His party would have to win both houses to form a stable government, and given the uncertainty of possible alliances, a clear picture of prospects for a new Italian government could take days. It is all but impossible that Bersani would team up in a ‘‘grand coalition’’ with his arch-enemy Berlusconi.

Grillo’s camp also played down the prospect of cooperation with the ex-premier, who has been embroiled in sex and corruption scandals.

‘‘Dialogue with Berlusconi? It is very difficult to imagine that Berlusconi would propose useful ideas (for the movement),’’ said 5 Star Movement candidate Alessandro Di Battista at Rome headquarters. ‘‘It never happened until now, but miracles happen.’’

Italy’s borrowing costs have reflected the optimism that the country will stick to its reform plans.

The interest rate on Italy’s 10-year bonds, an indicator of investor confidence in a country’s ability to manage its debt, fell to 4.19 percent in afternoon trading Monday. Last summer, at the height of concern over Italy’s economy, that interest rate was hovering at about 6.36 percent.

Milan’s stock exchange closed slightly higher, with the benchmark FTSE MIB up 0.73 percent to 16,351 points.