Arauz conceded defeat after receiving 47.5 per cent of the vote compared to Lasso's 52.5 per cent, with 97 per cent of poll statements processed, according to the National Electoral Council, on Sunday.

In a speech to supporters, Arauz said he would call Lasso and offer his congratulations.

"It's a day in which all Ecuadoreans have decided their future, they have used their vote to express the need for change and desire for better days," said Lasso at a rally where jubilant supporters chanted "Lasso President".

His victory will be welcomed by foreign investors unnerved by Arauz's promises of extensive social spending in the face of weak government finances and a struggling economy.

It also bucks a trend of leftist victories in Latin America including in Argentina, Chile and Bolivia.

Lasso will take office on May 24.

The oil-exporting nation's economy was already weak due to low crude prices when the pandemic started.

It has pushed a third of the population of more than 17 million into poverty and left half a million people unemployed.

President Lenin Moreno, who did not seek re-election, imposed painful austerity measures as part of a $6.5 billion financing agreement with the International Monetary Fund, but was unable to kick-start the economy.

The election council figures show 1.6 million null votes, likely the result of indigenous activist Yaku Perez calling on supporters to spoil their ballots.

Perez ran in the first round vote in February and narrowly lost out to Lasso for a slot in the run-off, which he attributed to electoral fraud despite not presenting evidence.

Australian Associated Press