The vote is a test of the legacy of outgoing President Rafael Correa, Moreno's ally and an outspoken critic of the United States.
With 51.8 percent of the ballots counted, Moreno had 38.26 percent of the vote with 29.86 percent for his conservative rival Guillermo Lasso, the president of the National Electoral Council, Juan Pablo Pozo, said in a televised announcement.
Moreno needed more than 40 percent and a 10-point lead over his rival to win outright without facing a runoff.
- Trump -
If in the end Lasso wins the presidency, a pillar of the Latin American left will swing to the right.
Correa says Latin America needs a strong leftist movement to resist US President Donald Trump's hard line on immigration and trade.
But Lasso has shown more willingness to work with Washington since Trump's election victory in November.
- Assange -
Lasso has also said he will end WikiLeaks founder Julian Assange's asylum in Ecuador's London embassy.
Assange is taking refuge there for fear of extradition to the United States for publishing leaked documents that embarrassed Washington.
The busting of a commodities boom has hastened the end of two decades of leftist predominance in Latin America.
Argentina, Brazil and Peru have all switched to conservative governments since late 2015.
- Economic crossroads -
Correa says Sunday's presidential and legislative polls are an opportunity to halt the "conservative restoration" in Latin America.
The busting of a commodities boom has hastened the end of two decades of leftist predominance in the region.
Economist Correa, 53, initially oversaw a boom in the oil-rich country of 16 million.
Now, "what is at stake are two visions of society, two visions of development, two visions of the state," he said of Sunday's election.
Voters were deciding whether to continue his tax-and-spend policies or give Lasso a mandate to cut spending and taxes.
Lasso has slammed Correa's allies over alleged links to corruption.
- Boom, bust -
Ecuador exports half a million barrels of oil a day. Correa used the wealth to fund social welfare schemes and public works.
But oil prices have plunged over the past three years.
The economy shrank by 1.7 percent last year.
"I want a change because it has been stronger than any economic crisis we have felt before," said Alexandra Orbe, 48, before voting in the capital Quito.
But voter Nora Molina, 53, judged "these past 10 years have shown how the country has advanced. I think we are going to keep that going."
- Close contest -
Analysts said voters fed up with Correa may rally behind a conservative candidate if the vote goes to a second round.
"Any party could beat the governing one in the second round, because there is major resistance to and rejection of the government," said political scientist Paolo Moncagatta of Quito's San Francisco University.
But Michael Shifter of the Inter-American Dialogue think tank in Washington cautioned: "It is a mistake to underestimate the strength of support for Correa's side."
Voters also cast ballots in a simultaneous referendum on whether to fire politicians who hold funds in tax havens while in office.