The vote is a setback for the country's main political parties and many politicians who urged voters to reject the 105-page constitution, the country's 20th in 84 years.

Election commissioner Somchai Srisutthiyakorn told reporters that with 92 per cent of ballots counted 62 per cent of voters approved the constitution that was seen a test for the military government that seized power in a 2014 coup.

"The voting was smooth in all areas," he said.

Mr Somchai said 58 per cent of voters also approved a supplementary question on whether a military appointed Senate could choose an unelected prime minister.

About 55 per cent of 50 million eligible voters turned out to vote.

The junta, formerly known as the National Council for Peace and Order (NCPO) and led by former general Prayuth Chan-ocha, used sweeping powers to stifle debate and ban political rallies and independent campaigns against the constitution.

Mr Prayuth said the referendum was a voluntary initiative taken as part of his government's people-centred policy.

He said his government had listened to the people, no matter whether they came out to vote or not.

But he said it was disappointing there had been some "inappropriate interventions by foreign elements during these delicate times of our political transition."

More than 100 people who campaigned for a "no" vote were jailed, including student activists and politicians.

Jatuporn Prompan, leader of the anti-junta Red Shirt movement, hit out at the result.

"I want to tell Prayuth that your victory is nothing to be proud of as your opponents had no chance to fight," he told reporters.

"I am still convinced that power will return to the people some day."

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

Sunai Phasuk, a senior researcher at Human Rights Watch, said the referendum was not legitimate.

"This is a redo of a military coup, using fear and intimidation to force Thai people to grant an extension of their control of power," he said.

Some Thais going to vote were quoted in the Thai media as saying they did not understand what they were voting for.

The constitution shifts power to the judiciary and a Senate appointed by the military while effectively hobbling elected representatives with all kinds of 'checks' and balances.

The Senate will include commanders of the army and other security services.

Critics say the overall aim is to create coalition governments that are fragile and fractious and effectively extend the military's influence for the foreseeable future.

"The goal is to keep the political party system fragmented to bring about a coalition government that will not be dominated by a single party," said Thitinan Pongsudhirak, a political science professor at Bangkok's Chulalongkorn University.

Parties aligned with former telecommunications billionaire Thaksin Shinawatra have easily won every election since 2001, upsetting Bangkok's powerful royalist and military elite.

Mr Thaksin lives in exile to avoid jail for corruption but he retains a strong influence on many Thais, especially in the country's poorest provinces in the north and north-east.

Yuthaporn Issarachai, dean of political science at Sukhothai Thammathirat Open University, said the vote would lead to more compromising and negotiating in politics.

"It won't be politics ruled by the majority," he said.

"So we expect to see some adjustments from the political parties. We may see the switching of sides and some negotiations."

Both the country's main parties – the Democrats and pro-Thaksin Pheu Thai – declared their opposition to the charter in a rare moment of agreement between the bitter rivals.

Mr Thaksin described the vote as a "folly" that would perpetuate the junta's power and make it impossible to govern Thailand.

The referendum was the first chance for Thais to vote since the military overthrew the elected government of the country's first woman prime minister Yingluck Shinawatra in 2014.

Ms Yingluck is Mr Thaksin's younger sister.