The West sees the election as a key democratic test for the small landlocked nation of 2.9 million, which has no history of transfers of power to an opposition through the ballot box.
The vote is dominated by fierce competition between the ruling party of pro-Moscow President Serzh Sarkisian and an opposition coalition led by Gagik Tsarukian, a former arm wrestler who is one of the country's wealthiest businessmen.
Prime Minister Karen Karapetyan told journalists he "voted for progress and security," urging ballots be cast in "a spirit of solidarity and tolerance."
But opposition politicians reported violations at polling stations after previously warning that the government is preparing mass electoral fraud.
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"We have recorded numerous violations at polling stations - violation of ballots' secrecy and multiple voting," Hovsep Khurshudyan, a leader of Ohanyan-Raffi-Oskanyan, an opposition coalition, told AFP.
Armenia's interior ministry said it was probing 320 complaints of irregularities, while Justice Minister Arpine Hovhannisyan demanded parties "refrain from incorrect claims of mass violations."
US-funded Radio Free Europe/Radio Liberty said its correspondent Sisak Gabrielyan "was attacked by government loyalists in Yerevan today while covering Armenia's parliamentary elections."
Violence flared following Sarkisian's election in 2008 with 10 people killed in clashes between police and opposition supporters.
This time, the country aims to hold an exemplary vote to elect "a parliament trusted by society," the president told AFP in a March interview.
He said his government "has made enormous efforts so that (today's) milestone vote is flawless."
The amendments will shift the country away from a strong presidency to a parliamentary form of government after Sarkisian's second and final term ends in 2018.
The opposition says the changes were made to allow Sarkisian, 62, to maintain his grip on power by remaining party leader after he steps down as president.
Sarkisian denies that, saying the changes are "part of Armenia's democratisation process."
Ahead of the vote, Sarkisian told AFP he would remain "active" in politics after he left office by staying as party leader.
Both ruling and opposition parties have campaigned on populist promises such as "jobs, wages, pensions," Gevorg Poghosyan, a pollster at the Armenian Sociologists' Association, told AFP.
"That's what matters to the voters" in a country where about 30 per cent of the population live under the official poverty line, he said.
"The Republican Party and Tsarukian's coalition are likely to get the lion's share of seats in the new parliament - more than 80 per cent," Poghosyan predicted.
Tsarukian has built his campaign on lavish promises to cut tariffs on natural gas and electricity and hike public-sector salaries and pensions. He accuses the government of failing to address poverty and endemic corruption.
A party needs to clear a five-per cent threshold to win seats, while an electoral bloc made up of several parties needs at least seven per cent of the vote.
Voting was being monitored by international observers from the Organisation for Security and Cooperation in Europe.