Ponta conceded a surprise defeat as presidential candidate for his former communist Social Democratic party in Romania's runoff election on Sunday.
Ethnic German Klaus Iohannis (pictured) looked set to become president on a mandate to tackle corruption and turn Romania into a "normal" country.
Ponta told some 15,000 people outside his office:"I called Mr Iohannis and I congratulated him," adding however that as premier he intended to retain his post.
"I have no reason to resign [as premier]," Ponta told a local television station.
Protests over expatriate voting delays
Several thousand protestors had massed in the capital, Bucharest, to underscore complaints by expatriate Romanians of long queues and bungled poll organization abroad.
Romanian television had shown pictures on Sunday of long queues of people snaked outside polling stations in Paris, London, Vienna and other European cities.
The Liberal opposition leader said they had come out "of their houses to defend the right to vote," 25 years after the overthrow of communism.
"Thanks to you, another kind of Romania is beginning," Iohannis said on his Facebook page. "The Romania we want is not one of conflict or revenge."
Iohannis will replace Ponta's rival, President Traian Basescu, who after 10 years was excluded from a third term. Ponta and Basescu often feuded, prompting constitutional wrangles.
The post of president was created in 1974 under the late communist dictator Nicolae Ceausescu.
The president is in charge of foreign policy and defense, and names key public prosecutors and the chiefs of intelligence.
Iohannis has promised an independent justice system and to turn Romania into a "normal" country. Romania's justice system is currently under special EU supervision.
Reversal for Ponta
In Romania's first round election on November 2, Ponta had emerged with a 10-percent lead over Iohannis.
Official results were expected on Monday. Early figures indicated that around 285,000 Romanians voted abroad on Sunday - around twice as many as on November 2.
Sunday's turnout, put at about 61 percent among 18 million eligible voters, was among the highest in any Romanian election over the past two decades.
Sunday's runoff was widely seen as pivotal for democracy in one of the European Union's most troubled nations, which is also borders the flashpoint, Ukraine.
Romania is emerging from budget cuts imposed during the global slowdown, and growth rebounded in the third quarter of 2014. The country has a reputation for corruption and tax evasion.
Expat vote could be decisive
Sunday's close prognoses had placed the focus on returns from Romanian expatriates, who account for 3 million of the 18 million eligible voters.
Romanian authorities said voting procedures had been improved. Romanians living abroad were required to vote in their countries of residence.
Expatriates traditionally vote conservative, said Christian Ghinea of the Romanian Center for European Politics. If the voters were mobilized, then Iohannis had a chance, he told the AFP news agency.
Sergiu Miscoiu, an analyst with the CESPRI political think tank, said Ponta had made a "major mistake" by ostracizing expatriate Romanians.
"The main result of that was not necessarily the diaspora vote, but the solidarity with it in Romania," Miscoiu said in an interview with Reuters.
ipj/msh (dpa, AFP, AP, Reuters)