Italy's parliamentary elections have ended in stalemate and the possibility of a hung parliament.
With all domestic votes counted, Pier Luigi Bersani's centre-left bloc won the lower house vote but has failed to secure a majority in the Senate.
News of the results led to a sharp fall on Italian financial markets.
Mr Berlusconi conceded the lower house vote but control of both houses is needed to govern. A protest movement led by comedian Beppe Grillo won 25%.
Meanwhile a bloc led by current Prime Minister Mario Monti came a poor fourth, with about 10%.
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 centre-left leader Pier Luigi Bersani as the last of the votes were being counted.
Mr Berlusconi conceded to his opponents in the lower house. He said that everyone should now reflect on what to do next, but fresh elections should be avoided. He would not do a deal with Mr Monti's centrist bloc, he added, saying that the prime minister's poor showing was down to popular discontent with his austerity measures.
With returns from all polling stations processed, the interior ministry figures gave Mr Bersani's centre-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.
But the winning bloc is guaranteed 340 seats, giving it an automatic majority.
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 centre-right bloc was expected to emerge with a higher number of seats.
'Jump to nowhere'
The results produced a mixed reaction in the rest of Europe.
German Foreign Minister Guido Westerwelle urged Italy to continue its reforms, and called for a government to be formed "as quickly as possible".
But his Spanish counterpart there was "extreme concern" about the financial consequences.
"This is a jump to nowhere that does not bode well either for Italy or for Europe," Garcia-Margallo said, quoted by Reuters news agency.
Markets fell as the outcome of the election became clear.
Italy's FTSE MIB index fell 4.7%, while London's FTSE 100 shed 1.5% and share markets in Frankfurt and Paris also fell more than 2%.
In New York, the Dow Jones Industrial Average fell 1.55% and Asian markets lost between 0.7% and 2.2%.
The yield on Italian government bonds rose sharply, implying markets are more wary of lending to Italy.
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, making it the most popular single party in the lower chamber.
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.