CHAIYAPHUM (THAILAND) - More than 2.2 million Thais turned out for early voting - almost 90 per cent of those registered - for the country's hotly anticipated national election, as the junta's main opposition predicted a cliffhanger in the first polls in eight years.

The 2011 election had a 55 per cent turnout for advanced voting, bringing about 1.4 million early to the ballot box.

But enthusiasm is soaring from a public starved of democratic processes since a 2014 coup ousted then Premier Yingluck Shinawatra - reflecting in the country's high number of early voters on Sunday.

There was an 86.98 per cent turnout among the 2.6 million early voters registered, Mr Ittiporn Boonpracong, chairman of the Election Commission (EC), said on Monday.

Many voting centres had "more than 90 per cent turnout".

Since the junta's takeover in 2014, leader Prayut Chan-o-cha has repeatedly postponed elections, seeding resentment among a public weary of crackdowns on dissent and political debate.

With election day looming, voter enthusiasm is writ large across the country and online, with Thais sharing memes and engaging in debate.

A high voter turnout could likely spell trouble for the junta-aligned Palang Pracharath Party, which is fronted by Mr Prayut as their prime ministerial candidate.

Mr Attasit Pankaew, a political scientist at Thammasat University, said there was a possibility of total voter turnout hitting 80 per cent on March 24, well above the 70-75 per cent turnout seen in 2011.

"People are possibly more alert and conscious of the election because of all the news," he said.

"And they are more determined and will not easily back down because they have learnt about the possible difficulties beforehand. So, they are unlikely to be deterred by long queues and crowds."

More than 7 million young Thais will be voting for the first time, making them wildcards in a country used to neat political categories.

Pheu Thai - the country's most popular party which brought Yingluck to power in 2011 - remains confident about its chances, said Ms Sudarat Keyuraphan, its prime ministerial candidate, during a campaign stop in the north-eastern province of Chaiyaphum on Monday.

"Yesterday's early voting saw people stand under the hot sun for three hours.... they want to say it is time up for the dictatorship," she told Agence France-Presse.

But the election's outcome will be difficult to predict under a junta-scripted Constitution, which analysts say was designed to undermine a popular party like Pheu Thai.

"It is not easy for us to win... but it is not easy for them (Palang Pracharath) either," Ms Sudarat said.

Thousands of supporters cheered wildly as Ms Sudarat and other party executives promised to raise rural incomes - a sign of Pheu Thai's enduring dominance in the country's north-east.

First-time voter Sroi, 30, said the slumping sugar cane prices during the junta's rule has prodded her to become more politically active.

"I will definitely vote this time... for Pheu Thai. They have good agricultural policies," she said.

Amid criticism and questions about the conduct and transparency of the early voting, the EC's Mr Ittiporn said the authority had found only three cases of electoral laws being violated.

However, there have been reports of people being given the wrong ballots, and netizens have posted that when they drew the attention of station officials to the mistake, they were told to use the incorrect ballot.