Armenians started voting in landmark legislative elections for the first time since the adoption of constitutional reforms aimed at transforming the ex-Soviet country into a parliamentary republic.

Sunday's election is expected to be a close race between the majority Republican Party of Armenia, backed by President Serzh Sarkisian, and an alliance of businessman and former world champion arm wrestler Gagik Tsarukyan's Prosperous Armenia party.

The poll election is 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.

But the campaign has already been marred by opposition claims that the government is preparing mass electoral fraud.

Before the vote, the European Union delegation to Armenia and the US embassy said in a joint statement that they were "concerned by allegations of voter intimidation, attempts to buy votes, and the systemic use of administrative resources to aid certain competing parties".

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There are also fears of violence after 10 people were killed in 2008 clashes between police and opposition supporters following the election of pro-Moscow President Sarkisian.

This time, the country aims to hold an exemplary vote to elect "a parliament trusted by society," the president told AFP news agency in an interview in March.

He said his government "has made enormous efforts so that (Sunday's) milestone vote is flawless".

The polls come after constitutional amendments initiated by Sarkisian in 2015 that his opponents say were designed to keep the ruling Republican Party in power.

The changes were passed after a referendum, but they also prompted thousands to rally in protest.

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.

Two decades in power

The opposition alleges that the changes were made to allow Sarkisian, 62, to maintain his grip on power by remaining party leader after he steps down as president.

"The amendments will perpetuate the rule of Sarkisian and his Republican Party," which has held onto power for two decades, said Aram Manukyan, an MP from the Armenian National Congress opposition party.

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Sarkisian has denied the allegations and defended the changes as "part of Armenia's democratisation process," saying they would empower the opposition.

In advance of the vote - in his first comments on his political future - Sarkisian said that he would remain "active" after he left office and hinted that he would keep influencing Armenia's politics as leader of the Republican Party.

"When one is leader of a big political party, the scope of one's responsibility and duties increase," he said.

"As chairman of the Republican Party, I assume responsibility for my teammates," he said when asked about his post-2018 future.

A total of five parties and four electoral blocs are running in Sunday's vote, with 101 parliamentary seats up for grabs under a proportional representation system.

A party needs to clear a threshold of five percent to be represented in parliament, while an electoral bloc made up of several parties needs to garner at least seven percent of the vote.

Voting, which started at 04:00 GMT and ends at 16:00 GMT, will be monitored by international observers from the Organisation for Security and Cooperation in Europe.