A Moreno victory would come as a relief for Wikileaks founder Julian Assange after Guillermo Lasso vowed to remove him from the Ecuadorean embassy in London if he won the runoff.

Moreno, a paraplegic former vice-president, had secured 51.07 percent of the votes compared to Lasso's 48.93 percent, with just over 94 percent of votes counted, according to the electoral council. It has not yet declared a winner.

Right-leaning governments have come to power in Argentina, Brazil, and Peru recently as a commodities boom ended, economies flagged and corruption scandals grew. Lasso, a former banker, had promised to denounce embattled Venezuelan President Nicolas Maduro, an ally of Ecuador's current government.

A bitter Lasso disputed the results on Sunday night.

"They've toyed with popular will," he told Ecuadorean television on Sunday night, asking for a recount and repeating he was the real winner of the vote.

He cited the first round of the election in February, when final results took days to come out and his supporters massed in front of the electoral council to guard against what they said were fraud attempts.

Hundreds of Lasso supporters swarmed in front of the electoral council offices in capital Quito and coastal city Guayaquil, Lasso's hometown, chanting "No to fraud" and "No to dictatorship!"

'LENIN PRESIDENT!'

Moreno, who has been in a wheelchair since losing the use of his legs two decades ago after being shot during a robbery, would become one of the world's rare presidents to use a wheelchair if he takes office on May 24.

"Lenin," as he is commonly referred to by his supporters, was already celebrating a victory that would extend a decade of leftist rule.

"From now on, let's work for the country! All of us!" Moreno told flag-waving supporters in the mountainous capital Quito.

A former UN envoy on disability, he has a more conciliatory style than fiery outgoing President Rafael Correa and has promised benefits for single mothers, the elderly, and disabled Ecuadoreans.

He would face strong pressure to create jobs amid an economic downturn and crack down on graft amid corruption scandals at state-run oil company PetroEcuador and Brazilian conglomerate Odebrecht.

Lasso has criticized Moreno as ill-equipped on the economic front and warned his major social promises would worsen Ecuador's already steep debts. Moreno's supporters, in turn, have decried Lasso's plans, warning that he would slash welfare benefits and govern for the rich.

The ruling Country Alliance on Sunday said results were irreversible.

"The revolution has triumphed again in Ecuador," tweeted Correa, who has said he will move to Belgium, where his wife is from, when he leaves office. "The right has lost, despite its millions and its media."