Among the first voters who cast their ballots was President Robert Kocharian.
Addressing reporters at a Yerevan polling station, he implicitly urged Armenian citizens to back changes that he described as democratic.
"Today, people in Armenia face this choice -- either to have more balance of power between the branches of government, or to keep a strong presidential system," Kocharian said.
To add weight to his remarks, Kocharian openly displayed his "Yes" ballot paper before television cameras.
Opposition parliamentarian Viktor Dalakian criticized the gesture as an infringement of the existing legislation.
"Kocharian has violated the Second Article of the Law on Referendums and Article Three of the constitution. These articles forbid open voting. By openly voting, the president is actually instructing others to vote 'Yes.' We regard this as a violation," Dalakian said.
In theory, the proposed changes are meant to transfer part of the president's prerogatives to the parliament, the government, and strengthen the judiciary. They will also allow millions of diaspora Armenians to obtain citizenship in the Southern Caucasus country.
The European Union, the Council of Europe's Venice Commission, and the United States have backed the proposed changes, saying they are vital to the reform process in Armenia. On paper at least, the changes are in line with the commitments Armenia undertook when it joined the Council of Europe in 2001.
Opposition leaders say Kocharian has no right to modify the constitution. They say that's because he came to power in 1998 and has consolidated his rule through rigged presidential and legislative polls. They have called for a boycott, claiming the changes in reality aim at strengthening Kocharian's powers
Yesterday, several hundred people demonstrated in central Yerevan to denounce the upcoming vote.
Defense Minister Serzh Sargsian -- whom many in Armenia see as a potential successor to Kocharian -- today ridiculed the opposition for failing to gather larger crowds against the referendum.
"If I organize any meeting and cannot have at least 1,000 people there in attendance, I would not commit suicide, of course. But I would definitely be very ashamed and unable to look into people's eyes," Sargsian said.
Regional political analysts believe apathy among Armenian voters is the greatest challenge faced by Kocharian and his political allies.
RFE/RL's Armenian Service reports that 200,000 voters have taken part in the vote as of 1200 local time. The polls close at 2000 local time.
To take effect, the constitutional changes must be endorsed by at least one-third of Armenia's 2.3 million registered voters and 50 percent of the actual voters.
To boost the turnout, the government earlier this month printed copies of the draft constitution, saying it would try to deliver them to every single family in the country.
Claiming the government will be unable to secure a high turnout, the opposition says it suspects authorities of seeking to rig the vote.
It has threatened to take to the streets in case of fraud.
Kocharian today said the referendum will be "free and fair" and pledged to respect any outcome of the vote.
Armenian President Robert Kocharian (file photo)
A PRESIDENTIAL SYSTEM OR A PARLIAMENTARY SYSTEM? The Armenian government has issued a pamphlet of frequently asked questions about the 27 November referendum in order to get the state's view across. To read a complete translation of this document, click here.