Thais began voting 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 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 favor of accepting the new constitution, but most voters were undecided. Preliminary results were expected at around 8.00 p.m. local time. 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.
He encouraged Thais to participate after casting his vote on Sunday.
"I urge everyone to come out and vote… to decide on the future of the country," Prayuth told reporters at a polling station in northwest Bangkok.
He was heckled by one woman as he spoke to the media.
"Can you manage the country? Can you protect the country?" she asked, before being moved away by security personnel.
Around 200,000 police were deployed for the vote, and while Thailand's largest political parties rejected the constitution ahead of the vote, there were no signs of protests or trouble.
The junta, formally known as the National Council for Peace and Order, has banned all criticism of the constitution and authorities have detained and charged dozens of people who have spoken against it, including politicians and student activists.
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.
Thailand has seen over a decade of political turmoil since Thaksin was ousted.
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 three years later in 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.
In the northeastern city of Khon Kaen, around 50 voters queued to vote outside the town's rebuilt city hall, which was burnt down during political unrest in 2010.
"I want the country to get better," said farmer Thongyoon Khaenkhaomeng at a nearby polling station in a school. He voted in favor of the constitution because he wanted to see an end to Thailand's divisions, he said.
Day laborer 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 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.
Whichever way the vote goes, the United Nations would like to see more dialogue between the military and political opponents, said Luc Stevens, the UN chief in Thailand.
"There is no reconciliation if one group says 'Lets reconcile on our terms'," he told Reuters ahead of the referendum.
"If you don't want to leave anybody behind in this country, you need to think about an inclusive process, an open dialogue, and ensure that people can express their opinion."
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, said Police Colonel Aram Prajit.
The ban on campaigning has not stopped the junta from deploying thousands of military cadets to carry a message to Thailand's 50 million eligible voters encouraging them to participate in the referendum. The Election Commission is hoping for a turnout of 80 percent.
Amnesty International said on Friday the junta had created a chilling climate ahead of the vote through pervasive human rights violations.