Euronews reported exit polls showed Sarksyan with 58 percent of the votes cast. Raffi Hovannisian was a distant second with 32 percent.
Election officials said Monday's polling was free of fraud overall, though Hovannisian alleged otherwise, ArmeniaLiberty.org reported.
Tigran Mukuchian, chairman of the Central Election Commission, said before the polls closed there had been only "isolated cases" of irregularities reported by media and opposition candidates.
"Those alerts that are now being investigated relate to isolated cases observed on Election Day," he told RFE/RL's Armenian service.
Hovannisian alleged "numerous" violations such as vote buying, multiple voting and voter intimidation.
"I don't remember seeing violations of this scale and type before," Hovsep Khurshudian, his campaign spokesman, said at a news conference.
Unlike 2008 when 10 people died in Election Day clashes, there were no immediate reports of violence Monday, Euronews said. Minor party candidate Paruyr Hayrikyan, 63, was shot in the shoulder last month.
Another challenger, political commentator Andreas Ghukasian, was on a hunger strike demanding Sargsyan be removed from the ballot.
The New York Times noted the 58-year-old Sargsyan, who has guided Armenia to modest economic improvements in recent years, had been the predicted winner for months.