Ecuadoreans are growing impatient at the slow pace of vote counting with the results of the presidential poll delayed until Wednesday or Thursday.
It is still not clear whether there will be a second round or if governing party candidate Lenin Moreno will gain the 40% of the votes needed to win outright in the first round.
With 95% of the votes counted, he fell just short with 39.23%.
His closest rival, Guillermo Lasso, said the delays "did not smell right".
Mr Lasso is trailing more than 10 percentage points behind Mr Moreno but is confident he could beat him in a second round in which opposition supporters are likely to unite behind him.
National Electoral Council President Juan Pablo Pozo said there was a "marked trend" and that a run-off looked "inevitable" with only 5% of votes still to be counted.
But he called on Ecuadoreans to "wait for the results with calm".
He assured Ecuadoreans that "not one vote will be stolen from anyone".
- 63-year-old left-wing former vice-president
- Close ally of outgoing President Rafael Correa
- Became paraplegic after being shot in the back in 1998
- Wants to increase employment opportunities and give all Ecuadoreans the chance to go on to higher education
- 61-year-old former banker
- Youngest of 11 children
- Wants to create a million jobs by promoting foreign investment and has promised to cut taxes for big companies
- Has vowed to evict WikiLeaks founder Julian Assange from the Ecuadorean embassy in London if he wins
Third-placed candidate Cynthia Viteri has already said she would throw her weight behind Mr Lasso in a run-off.
"How can they take three days to count 12%?" Mr Lasso asked referring to the unprecedented slow pace at which the last remaining votes were trickling in.
In previous elections, official results were announced on the night of the election.
'Revolutionary for life'
Supporters of Mr Lasso, some of whom gathered outside the National Electoral Council on Monday, were not convinced.
"It seems like democracy in this country has disappeared and this government does whatever it likes with our vote," Mateo Mendez told Reuters news agency.
"It's very strange that here in the 21st Century the results aren't known the same day as the election," Mr Lasso said while calling on his supporters to stay mobilised.
Mr Moreno was critical of Mr Lasso's call for Ecuadoreans to take to the streets if there was no second round.
"It's striking to me that there is a loser politician out there calling for violence,'' he said.
Mr Moreno has said that while he is confident he will gain enough votes to stave off a second round, he is ready to carry on fighting.
"A revolutionary is a revolutionary for life," he tweeted [in Spanish].