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Reformist opposition candidate Traian Basescu (search) won Romania's presidential runoff election, according to nearly complete returns Monday — a blow to the successors of the once-powerful communists who ruled for most of the period since the 1989 revolution.

His opponent, Prime Minister Adrian Nastase (search), conceded defeat after results showed Basescu had won 51.23 percent of Sunday's vote, compared with Nastase's 48.77 percent. The results were based on 98.76 percent of the ballots counted.

In a victory speech, Basescu pledged to fight corruption, restore press freedoms and prepare Romania to join the European Union (search) by 2007.

He said he would strengthen ties with the United States and Britain to guarantee Romania's security, while also seeking good relations with Russia and other former Soviet states.

"The top priority is to fight corruption," Basescu said, adding he would free state institutions from political interference and "put them to work on behalf of the citizens."

Nastase called Basescu to convey his congratulations.

"It is the decision of the Romanian people and I respect it," Nastase said.

Outgoing President Ion Iliescu, who has led Romania for 11 of the past 15 years and had supported Nastase, also called Basescu to congratulate him, said the president's spokeswoman, Corina Cretu. She added that Iliescu said the runoff elections were fair and that they confirmed Romania has a working democracy.

Early results from Sunday's election sent hundreds of Basescu supporters onto the streets in cities around Romania. The opposition is seen by many Romanians as less connected to the communists who ruled until the 1989 revolution, and less tainted by corruption and political foul play.

However, Nastase's Social Democracy Party had overseen a period of economic growth, and the opposition, in four years in power until 2000, proved unable to reform a system riddled with corruption.

By law, the president names a prime minister, who then needs to be approved by a vote in parliament. The president can dissolve parliament if it fails to approve a government within 60 days. In the parliamentary elections, Nastase's party won 189 of 469 seats, while Basescu's Justice and Truth Alliance won 161.

Neither has enough seats to form a majority.

On Monday, a small political party that initially had supported Nastase's Social Democrats deserted the losing candidate.

The Humanist Party issued a statement saying it "reaffirmed its political independence," adding that its deal with the ruling party was only an electoral alliance and not a governing pact.

Basescu, 53, said he wanted to form a government from his Alliance party, the Humanist Party and an ethnic Hungarian party, while avoiding any dealings with an extreme nationalist party criticized by foreign governments.

In an interview with the Romanian service of the British Broadcasting Corp., he said he would not form a government with the nationalist Greater Romania Party or ally himself with its leader, Corneliu Vadim Tudor.

Basescu had campaigned for economic reform, promising to lower taxes and fight corruption. He is also seen as a social reformer and has said he supports greater rights for gays — a stance that drew heavy criticism from the Orthodox Christian Church.

Both candidates had supported Romania joining the European Union, which is expected to occur Jan. 1, 2007.