Armenia's election commission said more than 93 percent of those who voted approved the amendments, which include changes on the division of government powers. Voter participation was reported at 65 percent.
But opposition politicians said turnout was far lower. Vazgen Manukian of the National Democratic Union accused officials of rigging the vote.
"Two unprecedented things in my memory happened in Armenia today. Both were expected, but both were unprecedented. Never before had polling stations been so empty," Manukian said. "The authorities also showed impudent cynicism in the way they rigged the elections. In previous years we never experienced such blatant cynicism. In my opinion it is a national, state- level betrayal."
Government officials have defended the results of the referendum.