Italy faces political deadlock after the anti-establishment 5-Star Movement of comic Beppe Grillo become the strongest party in the country, leaving no group with a clear majority in parliament.
The centre-left coalition led by Pier Luigi Bersani won the lower house by around 125,000 votes and claimed the most seats in the Senate.
He was short of the majority in the upper house that it would need to govern.
Mr Bersani claimed victory but said it was obvious that Italy was in "a very delicate situation".
Party officials said the centre-left would try to form a government but it was unclear what its options would be.
Former prime minister Silvio Berlusconi has indicated that he is open to an alliance between his centre-right coalition and the centre-left group.
In a television interview, Mr Berlusconi said: "Italy cannot be left ungoverned, we have to reflect."
He said all parties need to sacrifice something for the common good.
On the other hand, he explicitly ruled out a coalition with outgoing Prime Minister Mario Monti, and said his austerity policies had dragged Italy into recession.
Mr Berlusconi added that he was not worried about a negative market reaction to the vote or a possible increase in Italy's borrowing costs.
Mr Grillo, a comedian-turned-politician who previously ruled out any alliance with another party, has not showed any immediate willingness to negotiate.
Financial markets reacted nervously to the prospect of a government stalemate.
Italy's borrowing costs have come down in recent months, helped by the promise of European Central Bank support.
But the election result confirmed fears that it would not produce a government strong enough to implement effective reforms.
Mr Grillo's surge in the final weeks of the campaign threw the race open.
Hundreds of thousands turned up at his rallies to hear him lay into targets ranging from corrupt politicians and bankers to German Chancellor Angela Merkel.
In just three years, his 5-Star Movement has grown from a marginal group to one of the most talked about political forces in Europe.
The party is heavily backed by a frustrated generation of young Italians increasingly shut out from permanent full-time jobs.
Its result of 25.5% in the lower house was just ahead of the 25.4% for Mr Bersani's Democratic Party.
Mr Bersani's party ran in a coalition with the leftist SEL party and it won almost 8.7m votes overall, more than any other single party.