With 99.33 percent of votes counted by Ecuador's National Electoral Council (CNE), Lenin Moreno, from the ruling PAIS Alliance, is winning with 51.16 percent, against 48.84 percent for Lasso. "We won't cross our arms and stand by", he said on Twitter. The organization sent 77 observers to 480 polling stations across Ecuador, including at the electoral council's data center.
But Lasso persevered on Monday, holding a rally under the pouring rain in his coastal hometown of Guayaquil and vowing to keep battling what he said was fraud.
Boosted by high prices for its oil exports, Ecuador registered solid economic growth during the first eight years of Correa's presidency, before tipping into recession in 2015.
Both candidates in Ecuador's presidential election are greeting jubilant supporters and claiming victory in what's shaping up to be a nail-biter race.
He accused President Rafael Correa of trying to install an "illegitimate" government and called on his supporters to protest peacefully but firmly.
"We're going to defend the will of the Ecuadorian people in the face of this fraud attempt", he said. But his conservative opponent, Guillermo Lasso, plans to object to Sunday's vote - he says the numbers don't add up, citing an exit poll that had showed him in the lead.
Thousands of angry Lasso supporters shouting "fraud" broke through metal barricades and nearly reached the entrance of the electoral council's headquarters in Quito before being pushed back by police.
Leftist government candidate Lenin Moreno had claimed victory in Sunday's vote, bucking a shift to the right across South America as Lasso's supporters took to the streets in protest.
I don't know if there was actual fraud in Ecuador's election, but there certainly is enough smoke to merit an arson investigation.
Moreno, who lost the use of his legs two decades ago when he was shot during a robbery, would become one of few presidents in the world to use a wheelchair if he takes office on May 24.
QUITO, Ecuador (AP) - WikiLeaks founder Julian Assange is celebrating the results of Ecuador's presidential runoff with a blast at the losing candidate who had pledged to evict him from Ecuador's embassy in London. "He's absolutely convinced he is the president-elect", he said.
Around 8.30 p.m. local time (7.30 p.m. ET) Moreno declared himself the victor, but shortly that after a nongovernmental organization that had been monitoring the result said that it was a virtual tie.
A more stern-faced Moreno warned against pre-emptive celebrations by his rival and said that his supporters mobilized throughout Ecuador would make sure the results are expected.
Former banker Lasso had vowed to remove Assange from the embassy if he won the runoff.
Moreno called for dialogue with the opposition, saying: "We know how to hear the criticisms".