The Central Election Commission reported less than 24 hours after the close of polls that, with all votes counted, Republican Party candidate Sarkisian garnered nearly 53 percent of the vote, enough to avoid a potentially risky runoff.
His nearest rival, Ter-Petrossian, had just 21.5 percent, and former parliament speaker Artur Baghdasarian finished in third place, with 16.6 percent of the vote.
The Organization for Security and Cooperation in Europe's (OSCE) election monitors issued a lukewarm assessment of the vote, saying it was "mostly in line" with international commitments.
The self-nominated Ter-Petrossian is alleging widespread violations during the vote, including ballot stuffing and physical intimidation and physical assaults on his supporters.
Ter-Petrossian threatened ahead of the vote to take his grievances to the street, and his supporters gathered in the capital's central square, near the election commission's headquarters. The building is surrounded by metal barriers and was being guarded by riot police armed with truncheons and shields.
Ter-Petrossian first made his allegations of irregularities immediately after he cast his own ballot on February 20, saying, "a lot of bad things are taking place."
The authorities have denied any manipulation of the election process.
In its assessment, the OSCE's election-monitoring body, ODIHR, said the vote was "mostly in line with the country's international commitments, although further improvements are necessary."
"On the positive side, we found that the authorities have made a genuine effort to address the shortcomings that we have noted in previous elections," ODIHR spokesman Jens-Hagen Eschenbacher told RFE/RL from Yerevan. "But there is a need for improvement and, in particular, to tackle some of the concers we have found during these elections -- and that includes continued lack of public confidence in electoral process, the absence of clear separation between state and party functions, and also we found that counting of the votes did not contribute to reducing suspicions among stakeholders in these elections."
Eschenbacher said the vote-counting process was asessed negatively at about 16 percent of the polling stations visited by mission observers.
Problems encountered at those stations included the inconsistent determination of valid votes, the use of mobile phones during the counting process, failures to post result protocols publicly, and even, in some cases, deliberate attempts to falsily the data entered into the protocol.
OSCE/ODIHR is expected to issue its final report on the elections in a few weeks.
"Our mission will remain in the country to observe the completion of tabulation process, and also the process of resolution of election complaints," Eschenbacher said. "That will be reflected in the final report."
Incumbent President Robert Kocharian, who was vocal in his backing of Sarkisian during the campaign, has already congratulated the prime minister on his victory, saying the vote was "free and fair."
Among the also-rans, Armenian Revolutionary Federation-Dashnaktsutiun candidate Vahan Hovannisian finished with 6.2 percent of the vote, former Prime Minister and National Democratic Union Chairman Vazgen Manukian received 1.5 percent, and the remaining four candidates polled under 1 percent.
Voter turnout was estimated at 69 percent, compared with 62 percent during the first round of the country's last presidential election, held in 2003.