BANGKOK/KHON KAEN, Thailand, Aug 7 -- Voters in Thailand headed to the polls on Sunday in a referendum on a new junta-backed constitution that would pave the way for a general election in 2017 but require future elected governments to rule on the military's terms.

The ballot is the first major popularity test for the junta led by Prime Minister Prayuth Chan-ocha, who has suppressed political activity during the two years since he seized power in a 2014 coup.

Ahead of the referendum, polls suggested a small lead in favour of accepting the new constitution, but most voters were undecided. Preliminary results were expected at around 8.00 p.m. (1300 GMT).

The junta has said the constitution is designed to heal more than a decade of divisive politics in Thailand that has dented growth and left scores dead in civil unrest.

Critics, among them major political parties, say it aims to enshrine the military's political role for years to come.

Prayuth has said he will not resign if Thailand rejects the constitution and that an election will take place next year no matter what the outcome.

"I urge everyone to come out and vote... to decide on the future of the country," Prayuth told reporters after casting his vote at a polling station in northwest Bangkok early on Sunday.

Voting through mid-afternoon had been trouble-free, said Police Major General Piyaphan Pingmuang.

"There have been no incidents to indicate protests or any political movement today," he said. "Everything is smooth."

Around 200,000 police were deployed for the vote.

Of 21 cases of voters tearing ballot papers, some were deliberate and others accidental, said Boonyakiat Rakchartcharoen, deputy secretary-general of the Election Commission.

Two students were detained and charged on Saturday in the northeastern province of Chaiyaphum for handing out leaflets urging voters to vote against the referendum, police said.

The junta, formally known as the National Council for Peace and Order, forbid debate about the constitution and banned campaigning ahead of the vote. The authorities have detained and charged dozens of people who have spoken against it, including politicians and student activists.

Amnesty International said on Friday the junta had created a chilling climate ahead of the vote through pervasive human rights violations.

The vote comes against the back-drop of concern about the health of King Bhumibol Adulyadej, 88. The military has for decades invoked its duty as defender of the deeply revered monarch to justify its interventions in politics.

Decade of turmoil

Critics say the charter is the military's attempt to make good on their failure to banish former Prime Minister Thaksin Shinawatra and his brand of populism from Thai politics after the coup that removed him in 2006.

Thaksin lives in self-imposed exile but retains a strong influence, particularly with his rural support base in the north. His sister Yingluck swept to power with an electoral landslide in 2011, and her government was ousted by Prayuth n the 2014 coup.

Yingluck, who was banned from politics for five years in January 2015 after a military-appointed legislature found her guilty of mismanaging a rice scheme, also voted on Sunday.

"I'm happy that I could still exercise my rights as a (Thai) person," Yingluck told reporters, urging others to go and vote.

Thaksin called the charter a "folly" on Thursday, saying it would perpetuate the junta's power and make it impossible to govern Thailand.

Reuters interviews with senior officers showed the military's ambition is to make future coups unnecessary through the new charter by weakening political parties and ensuring the military a role in overseeing the country's economic and political development.

Under the constitution, which would be Thailand's 20th since the military abolished an absolute monarchy in 1932, a junta-appointed Senate with seats reserved for military commanders would check the powers of elected lawmakers.

"I want the country to get better," said farmer Thongyoon Khaenkhaomeng at a polling station in a school in the northeastern city of Khon Kaen.

He voted in favour of the constitution because he wanted Thailand's divisions to end, he said. Nearby, voters queued to cast their ballot at the rebuilt city hall, which was burnt down during political unrest in 2010.

Day labourer Decha Shangkamanee said he had voted against the charter because he disliked the junta, but did not expect the referendum to make much difference.

"I know that nothing really changes today with the way the country is ruled," he said.

The campaign ban did not stop the junta from deploying thousands of military cadets to encourage Thailand's 50 million eligible voters to participate in the referendum. The Election Commission is hoping for a turnout of 80 percent.