The candidate of the ruling party in Honduras has conceded Sunday's presidential election to his opposition rival who is now poised to become the country's first female president.

Nasry Asfura Tuesday congratulated Xiomara Castro on her victory in a video broadcast on local television. The move will end his conservative National Party's 12-year hold on power.

Castro led her leftist Libre Party to a preliminary victory, supported by an almost 20 percentage points lead over her opponent. With over 52 percent of votes tallied by Tuesday evening, Castro had 53.4 percent support, and Asfura 34.1 percent. But the election result had been held in suspense by a sluggish vote count at the electoral council. 

Minutes after Asfura's concession, U.S. Secretary of State Antony Blinken also congratulated Castro and said Washington looked forward to working with her.

"We congratulate Hondurans for the high voter turnout, peaceful participation, and active civil society engagement that marked this election, signaling an enduring commitment to the democratic process," Blinken said.

The concession brings to an end a turbulent period under the National Party, which has been dogged by scandals and corruption accusations, especially during the two terms of outgoing President Juan Orlando Hernandez.

Hernandez is deeply unpopular and has been implicated in a drug trafficking case in a U.S. federal court. He denies wrongdoing, but could face an indictment when he leaves office.

Castro's victory will see the left return to power after a 12-year hiatus that followed the ousting of her husband, former President Manuel Zelaya, in a coup in 2009.

Castro faces big challenges in Honduras, where joblessness, crime, corruption and the threat of transnational drug gangs have helped spur record migration to the United States.

She managed a strong showing in Sunday's election despite findings by the European Union vote observer mission that the National Party had used state backing to boost its campaign.

The smooth transmission of early election results had aided transparency and confidence, the EU mission said. But it criticized pre-election political violence and "abuse of state resources," such as a rise in handing out of welfare vouchers.

"The state media visibly favored the ruling party and its presidential candidate," said the mission's head, Zeljana Zovko.

Castro's team is already preparing for government. Hugo Noe, head of the campaign's policy platform, told Reuters Castro will seek to negotiate a new debt deal with the International Monetary Fund when she takes office in January.

(With input from Reuters)