The outcome of the election will shape how the South American country tackles its economic woes, including high inflation, a central bank running precariously low on dollars and a sovereign debt default.
Scioli promises he will make only gradual changes to outgoing leader Cristina Fernandez's brand of leftist populism while main pro-business opposition candidate Mauricio Macri talks of swiftly dismantling her protectionist controls to open up the ailing economy.
"There will be a run-off according to our data based on exit polls," said Marcos Pena, Macri's chief campaign strategist.
The electoral authority said early provisional results might not be known until 11 p.m. (2200 ET).
To win outright in the first round, Scioli needs 45 percent of votes or 40 percent and a 10 percentage point lead over the nearest rival. If he falls short of that, the two leading candidates will face off in a second round next month.
Alberto Perez, a central figure in Scioli's inner circle, told reporters that unofficial numbers showed the candidate won a "resounding victory" but he did not say if it was big enough to avoid a second round.
Former powerboat champion Scioli draws much of his support from poorer Argentines who credit Fernandez and her late husband and predecessor Nestor Kirchner with an expansive welfare system and lifting the economy out of a devastating 2001-2002 depression.
"I want Scioli to win. Macri won't care about the people. He has no idea what it is to be poor," said Carolina Carrizo, 28, from the town of San Justo outside the capital, whose family has lived off benefits since her husband hurt his back.
"I like how the country is doing at the moment and I think
Macri will reverse the progress."
Divisive legacy
Fernandez will step down with approval ratings near 50 percent, yet her eight years in power have been deeply divisive.
Pre-election polls showed that Macri, the conservative mayor of Buenos Aires and former president of the popular Boca Juniors soccer club, has stronger support among the urban middle and upper classes.
Macri is also favored by investors who are interested in Argentina's vast energy and metal reserves but are deterred by
the stifling controls that Fernandez has placed on the economy.
He and centrist Sergio Massa, who was in third place in pre-election polls, both promised to work quickly on dismantling trade and currency controls, and to improve the accuracy of government economic data that experts say is often fudged. They also vowed to tackle rising crime.
"I'm fed up with the insecurity and of the government giving hand-outs to delinquents who don't work," said saleswoman Florencia Corbalen, echoing a common refrain among the middle-class that Fernandez's welfare benefits have been too generous.
All three main candidates have tip-toed around the subject of negotiations with US creditors whose legal battle over unpaid debt tipped Argentina back into default last year. But each team says their candidate wants a deal that does not sell Argentina short.
The electoral authority said voter turnout was 79 percent. Voters also elected some regional governors, half of the lower house of Congress and a third of the Senate on Sunday.
Macri's camp said it was competing "vote for vote" against the ruling party in Buenos Aires province, home to more than one in three Argentines and where the ruling party had hoped to install a close Fernandez ally.
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