Italy is facing weeks of political uncertainty, with Prime Minister Silvio Berlusconi refusing to accept Romano Prodi's election victory.
Silvio Berlusconi says he wants all the proper legal checks
The prime minister has insisted on waiting until some 43,000 contested ballot papers have been re-examined.
He said on Wednesday the elections had been marred by "much fraud" and said the result "has to change".
But his centre-left rival Romano Prodi said his victory would not be reversed and that Mr Berlusconi "has to go".
Mr Prodi said he had already begun talks on forming a new administration and tackling the Mafia would be an absolute priority.
However, no administration can be sworn in for another month, until parliament has elected a new president.
The current head-of-state, Carlo Azeglio Ciampi, is due to step down then and he has made it clear he wants his successor to preside over the change of government.
Mr Prodi, who rejected Mr Berlusconi's suggestion of forming a grand coalition, said: "I have absolutely no fear of a reversal of the situation. It's a completely tranquil victory."
Mr Prodi was declared official winner of the parliamentary election after an extremely close race.
Mr Berlusconi, who heads a centre-right bloc, said there had been irregularities in the vote - particularly in the votes of Italians abroad.
"We won't hesitate to recognise the political victory for our adversaries, but only once the necessary legal verification procedures have been completed," said Mr Berlusconi.
Mr Prodi's supporters have criticised Mr Berlusconi as "irresponsible" for refusing to admit defeat.
Official results showed Mr Prodi had won just enough seats to control the Senate (upper house) after having already won a lower house majority.
The count gave Mr Prodi 158 Senate seats, against 156 for Mr Berlusconi. The interior ministry insists the results must still be confirmed by Italy's highest court, and that parliament's election committees would have to rule on any challenges.
The final results came after a nail-biting night of conflicting forecasts, based on exit polls and partial counts, which variously put the coalitions of Mr Prodi and his rival ahead.
The leader of the observation team from the Organisation for Security and Co-operation in Europe, Peter Eicher, said there appeared to be irregularities including "a very large number of blank, invalid or contested ballots".
He did not accuse either side, but said he also had misgivings about the late adoption of new electoral laws and unbalanced coverage by parts of the Italian media.
The coalition led by Mr Prodi - a former Italian prime minister and president of the European Commission - is thought to have won the vote in the lower house by just 25,000 votes.
But changes to the electoral system meant whoever won the lower house would automatically get a working majority there, even if the margin was small.