umbrella group of local monitors said their observers noted an "unprecedented number of violations of the electoral law and international standards."
The Armenian National Congress opposition party of former president Levon Ter-Petrosian boycotted a parliamentary session on Monday to protest against the alleged "falsification of the referendum results".
And a top opposition leader, Raffi Hovannisyan, called on the president and prime minister to resign.
"What happened today is state treason," he told supporters at a small rally in the early hours of Monday.
Prime Minister Ovik Abrahamyan promised that the "alleged electoral irregularities will be investigated and -- if confirmed -- those responsible will be prosecuted."
A shrewd former military officer, Sarkisian, 61, has been in charge of the small landlocked nation of 2.9 million since winning a vote in 2008.
After that poll, 10 people died in bloody clashes between police and supporters of the defeated opposition candidate.
Sarkisian won a second term in 2013. Earlier this year, brief protests erupted over electricity price hikes in the poor country, which has been badly hit by an economic crisis in Russia.
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