Leftist government candidate Lenin Moreno claimed victory in Ecuador's presidential vote on Sunday, bucking a shift to the right across South America, but the conservative challenger asked for a recount as his supporters took to the streets in protest.

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, secured 51.17 per cent of the votes, compared with Lasso's 48.83 per cent, with over 99 per cent of votes counted, the electoral council tally showed on Monday afternoon.

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.

Ecuadorean presidential candidate Guillermo Lasso, right, seen Sunday with his running mate Andres Paez in Guayaquil, has demanded a recount. (Henry Romero/Reuters)

A bitter Lasso, who had earlier proclaimed himself victorious based on a top pollster's exit poll, disputed the results that would extend a decade-long leftist rule in oil-rich Ecuador.

"They've crossed a line," he told supporters amassed in a hotel in his coastal hometown of Guayaquil, asking for a recount and vowing to challenge the results.

"We're going to defend the will of the Ecuadorean people in the face of this fraud attempt."

Lasso contrasted Sunday's fast results with the first round of the election in February, when a final tally took days to come out and his supporters gathered in front of the electoral council to guard against what they said were fraud attempts.

Lasso supporters protest

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!"

Police in Guayaquil used tear gas against supporters who were trying to break through a security cordon to the electoral council office there, according to a Reuters witness.

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.

Supporters of Ecuadorean presidential candidate Guillermo Lasso demonstrate near riot police Sunday night in Quito. (Mariana Bazo/Reuters)

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!" he told flag-waving supporters, flanked by running mate and current vice-president, Jorge Glas, and a beaming outgoing President Rafael Correa, before breaking into several songs.

A former UN envoy on disability, he has a more conciliatory style than the fiery 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.

Ecuadorean presidential candidate Lenin Moreno, a paraplegic after a 1998 shooting, celebrated with supporters at a Quito hotel. (Mariano Bazo/Reuters)

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 across a nation that stretches from Andean plateaus to the Galapagos Islands

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."