Victor Ponta had been the favorite to win, but was narrowly edged out by Klaus Iohannis, the mayor of the city of Sibiu who promised a crackdown on corruption.
Ponta yesterday said he had personally congratulated Iohannis. "We are a democratic country," Ponta said outside the headquarters of his Social Democratic Party two hours after polls closed. "The people are always right."
Ponta called on some 15,000 protesters gathered outside his offices to listen to his message in an apparent attempt to defuse tensions difficulties encountered by expat Romanians voting abroad.
About 300,000 Romanians who live overseas voted yesterday, many against the government. There were protests that they had been unable to vote in the Nov 2 election that led to the runoff.
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"Romanians, you were heroes today," Iohannis said before Ponta's concession, calling on authorities to count the vote correctly. "The vote was phenomenal!"
Iohannis praised voters, saying they had come out "of their houses to defend the right to vote" 25 years after the Romanian revolution.
Some Romanians waved toothbrushes to protest long waits at polling stations abroad and thousands protested in Munich, according to Mediafax news.
The government said it had improved the voting procedure this time at facilities, including Romanian embassies.
Ponta led by 10 percentage points in the Nov 2 vote, and corruption probes of senior Ponta aides appeared not to have dented the 42-year-old former prosecutor's chances.
Iohannis, 55, has promised an independent justice system if he becomes president. The winner will replace President Traian Basescu, who is stepping down after 10 years.