Three exit polls have predicted the result that would force a runoff vote.
Noboa won 28.5 percent of the vote compared to 26.5 percent for Correa, according to a Informe poll broadcast by Teleamazonas television.
A Market polling firm survey gave Noboa 28.23 percent compared to 27.17 for Correa.
Cedatos-Gallup had 27.6 percent for Noboa, with 25.5 percent for
Correa.
The runoff election is scheduled for November 26.
Result predicted
Opinion polls had predicted Rafael Correa, a 43-year-old economist and an ally of Venezuelan President Hugo Chavez, would virtually tie with banana magnate Alvaro Noboa, 56, the country's richest man.
According to the survey firm Market, Correa polled 28.4 percent against 27 percent for Noboa, well within the survey's margin of error.
Rafael Bielsa, head of an Organization of American States observation team, said the voting had gone ahead normally across Ecuador and despite small problems there was "no irregularity".
"No doubt there are problems typical in any electoral process in a
perfectly peaceful environment," he said.
"We have no reports of violence."
Bielsa, a former foreign minister from Argentina, has been slammed as biased by the leftist candidate, but Saturday he said charges of fraud were unproven.
Current President Alfredo Palacio said on Sunday that the election results "will be respected."
"Citizens can be certain that today they can go to the polls with a total
guarantee that their decision will be respected," he said at a ceremony marking the start of voting.
"Political reform in this country is continuing to take shape and the call for it has been heard around the country," he said.
Since 1979, only three democratically elected heads of state managed to serve out their full terms.