Romania's electoral bureau has rejected an opposition demand for parliamentary and presidential elections last Sunday to be annulled amid voter fraud claims.
Basescu says his fight is "to restore democracy"
The request was made by Traian Basescu, head of the Justice and Truth Alliance, who alleged that election authorities handed extra votes to his opponent.
Official results gave a narrow lead to the governing Social Democrats in both the polls.
European election observers have expressed concern over possible fraud.
The president of the electoral bureau, Emil Ghergut, admitted that there had been mistakes in the counting of spoilt ballots.
However, he said that all errors had been corrected and that the results of the election should stand.
The electoral bureau voted 21 to five to reject the opposition demand.
Speaking on Tuesday, centrist party leader Mr Basescu said that malpractice at Romania's Central Election Bureau gave around 160,000 extra votes to Adrian Nastase, the current Romanian prime minister.
With most votes counted, Mr Basescu's party trailed Mr Nastase by around 470,000 votes.
The challenger, who is currently mayor of Bucharest, also repeated claims made over the weekend that government supporters had been bussed between multiple voting stations in an organised fraud.
"We have no doubt this is fraud, [and] we want the immediate dismissal of election authorities," Mr Basescu said.
The vice-president of the Social Democrats (PSD), Miron Mitrea, insisted the election was fair.
"The elections were proper, despite some questionable practices," he said, quoted by the AFP news agency.
"There are some politicians who are sore losers and Traian Basescu is one of them," he added.
Final results in the parliamentary election, the source of most disputes, are expected on Wednesday evening.
"I am no longer [just] fighting for the presidency, but to restore democracy in Romania," he said.
Suspicion has focused on the introduction of a computerised electoral roll system for voters.
An expensive EU plan to introduce "foolproof" voting cards was suspended by the Romanian government before the election.
Instead, voters were allowed to vote at any polling station across the country.
Mr Basescu and observers from the Organisation for Security and Co-operation in Europe (OSCE) both claimed the system was vulnerable to manipulation.
Adrian Nastase's Social Democrats are accused of fraud
The electoral register of 18 million names - out of a total population of some 22 million - was declared suspect as concern mounted over the result.
The OSCE has said the vote appeared to be "professionally and efficiently organised" but officials have said they have not received an explanation for the use of a centralised electoral roll.
Stephen Nash, head of the OSCE mission to Romania, told the Associated Press: "In the context of a closely contested election, this has the potential to affect public confidence."
He urged that any allegations should be dealt with through "appropriate administrative and judicial processes."