Results based on more than 90 percent of ballots show that Thais have voted to approve a military-drafted constitution. Critics have called the charter undemocratic and warn it will entrench the junta's power.
Thailand's Election Commission announced that 61.5 percent of voters had backed the new constitution in Sunday's referendum, while 38.4 percent had rejected it.
The charter, drawn up by the junta, would lay the foundation for a civilian government influenced by the military and controlled by officials who have been appointed rather than elected.
The referendum was widely seen as a test for the popularity of the military government led by Prime Minister Prayuth Chan-ocha, a retired army general who came to power in a 2014 coup. He had promised to hold elections in 2017 after the constitution was passed.
Some 50 million people were registered to cast ballots in Sunday's poll, the first time Thais have been able to vote since the coup. Voting stations closed at 5 p.m. local time (1000 UTC), and more concrete results were expected later in the day.
Election Commissioner Somchai Srisutthiyakorn said turnout was only 55 percent.
The lead-up to the vote was marred by widespread international criticism over bans on campaigning and at least 195 related arrests. Opponents, including the country's major political parties, say the constitution will tighten the military's grip on power. The military, for its part, says the new document will curb political corruption and bring stability to the country.
The charter's most divisive clauses call for a junta-appointed Senate to check elected parliamentarians, and increased powers for Thai courts, already accused of political bias.
Another clause would make it easier to begin impeachment proceedings.
Climate of fear?
Amnesty International's regional deputy director, Josef Benedict, said the climate ahead of Sunday's vote was chilling.
"If people cannot speak their minds freely or take part in political activities without fear, how can they meaningfully engage in this referendum?" he said.
Another group, Thai Lawyers for Human Rights, said authorities had used a vaguely worded special law that effectively bans criticism of the charter.
The military deployed thousands of cadets to encourage eligible voters to participate in the referendum.
Since a 2006 coup, power in Thailand has flipped between elected governments led by or linked to self-exiled billionaire Thaksin Shinawatra and rule by an arch-royalist army and its establishment allies.