With all domestic votes counted, Pier Luigi Bersani's center-left bloc has narrowly beaten ex-PM Silvio Berlusconi in the lower house but has failed to secure a majority in the Senate, BBC reported.
Control of both houses is needed to govern and a Berlusconi official said the election was "too close to call".
A protest movement led by comedian Beppe Grillo won a quarter of the vote.
The outcome of the election, which comes amid a deep recession and tough austerity measures, was so close that the margin of victory given in interior ministry figures was less than 1% in both houses of parliament.
"It is clear to everyone that a very delicate situation is emerging for the country," said center-left leader Pier Luigi Bersani as the last of the votes were being counted.
Angelino Alfano, secretary of former Prime Minister Silvio Berlusconi's People of Freedom party, said the result was "extraordinary", and he urged the interior ministry to wait before declaring a final result.
The interior ministry figures were not official, he argued, and were "inevitably subject to a margin of error".
With returns from all polling stations processed, the interior ministry figures gave Mr Bersani's center-left bloc 29.54% of the vote for the lower house (Chamber of Deputies), barely ahead of the 29.18% polled by Mr Berlusconi's bloc.
Mr Alfano said the result was "too close to call" given the tight margin between the two blocs.
Votes cast outside Italy are still to be collected.
Mr Bersani also won the national vote for the Senate, but was unable to secure the 158 seats required for a majority.
As bonus seats are distributed in the upper house according to regional votes, Silvio Berlusconi's center-right bloc was expected to emerge with a higher number of seats.
Mr Berlusconi was heading for victory in three of the four big regions - Lombardy in the North, Campania in the center, and Sicily in the South.
Initial exit polls on Monday afternoon gave Mr Bersani's bloc a clear victory, prompting the Milan stock market to soar by nearly 4%.
But as the close result became clear the markets fell back. In New York, the Dow Jones Industrial Average fell 1.55% and shares in Tokyo opened sharply lower.
The apparent split between left and right in the eurozone's third largest economy is likely to cause great anxiety among leaders in other EU member states.
Mr Berlusconi, 76, left office in November 2011, facing claims of economic mismanagement as the eurozone struggled to contain Italy's debt crisis.
Italians have had more than a year of technocratic government under Mario Monti. But his attempts to reduce spending caused widespread public resentment and his decision to head a centrist list in the parliamentary elections attracted little more than 10% of the vote.
"Some supposed we'd get a slightly better result but I am very satisfied, we are very satisfied," he said.
In a surge in support, Beppe Grillo's anti-austerity Five Star Movement attracted 25.54% of the vote.
Correspondents say this was an extraordinary success for the Genoese comic, whose tours around the country throughout the election campaign - hurling insults against a discredited political class - resulted in his party performing well in both chambers.
"We've started a war of generations," Mr Grillo said in an audio statement on his website which taunted the leaders of the mainstream parties.
"They are all losers, they've been there for 25 to 30 years and they've led this country to catastrophe. "
As the extent of his success became clear on Monday night, Mr Grillo's supporters in his home town of Genoa celebrated early into Tuesday morning.
