Paraguay's ruling Colorado Party has won the presidency, extending a run of over half a century in power.
Duarte celebrated with his family
Nicanor Duarte Frutos won about 38% of the vote, with almost all votes counted, well ahead of his nearest rivals.
Paraguay's 2.4 million voters were also choosing a vice-president, a new parliament and 178 regional governors.
"We're going to fight so that Paraguayans can feel proud of their leaders and their country," said Mr Duarte, as results came in.
Julio Cesar Franco, of the Liberal party, polled 23%, with Pedro Fadul, a conservative businessman running on the Beloved Fatherland ticket, reaching 22%.
Unlike neighbouring Argentina, which also voted for a new president on Sunday, Paraguay does not require an absolute majority for victory in presidential elections, so Mr Duarte does not need to contest a second round run-off.
"I want to be a president that recovers the country's
credibility, a president that is respected by the international
community," said the new president-elect.
Mr Duarte, a 46-year-old lawyer and former education minister, joked that when he had been born the midwife told his mother that was so ugly he would probably be president.
Paraguay is one of Latin America's poorest countries. It is heavily reliant on agriculture and is notorious for smuggling.
The international watchdog Transparency International ranks it among the most corrupt countries in the world.
Mr Duarte's political opponents say that he has protected corrupt politicians.
Mr Fadul said that Mr Duarte had benefited from "use of the state machinery" to influence the state employees and their families, who make up 40% of the electorate.
Elections were held to replace the caretaker leader Luis Gonzalez Macchi, who did not stand in the polls.
Mr Gonzalez Macchi was appointed by parliament four years ago to replace the last elected president, Raul Cubas, who fled amid street protests.
Correspondents note that other Latin American states have responded to economic crises by ejecting entrenched political groups in recent elections.
Poverty was a key issue for many of the country's 2.4 million voters
But Paraguayans have stuck with Colorado, which has ruled for the past 60 years either as a civilian government or alongside military dictator General Alfredo Stroessner.
Mr Duarte is a proponent of free-market policies but has sworn not to privatise government services if elected. He has promised to crack
down on corruption and put the state's finances in order.
A Paraguayan sociologist, Bernardino Cano Radil, said the country suffered from a "cultural fear of change".