National referendum on Thailand's 2016 draft constitution and its additional question begins at 8 am on Sunday, while a general election in 2017 is promised by the present government irrespective of the outcome.
Major figures such as Prime Minister Prayut Chan-o-cha, former prime ministers Yingluck Shinawatra and Abhisit Vejjajiva have already cast their votes, Xinhua news agency reported.
The voting will close at all 94,000 polling stations at 4 pm, and the unofficial results will be known before 9 pm, according to the Election Commission.
This is the second referendum after 2007. Around 50.5 million Thais of the total population of 65 million are eligible to vote this time, compared to the 45 million eligible in the last referendum.
The Election Commission has set a goal of 80 per cent turnout, against the 57.61 per cent turnout in 2007.
Once passed, the draft constitution will become the 20th constitution since the country became a constitutional monarchy in 1932 and there will be a general election in 2017, according to the roadmap of the Junta's leader.
On the ballot there are two questions: "Do you accept the 2016 draft of the constitution?" and "In order to reform the country continuously according to the strategy, should the Senate be allowed to join the House of Representatives in the voting process to select a prime minister for the first five-year period after the first National Assembly is formed under this constitution?"
The result of the second question, if passed, will be added to the Transitory Provisions of the constitution and thus give 250 senators picked by the current junta, or the National Council for Peace and Order (NCPO), power to elect a Prime Minister along with 500 elected members of House of Representatives.
If the draft is voted down along with the additional questions, there has to be a new draft constitution before a general election, said the prime minister. He has already said the general election would be held next year regardless of the referendum outcome.
Suthep Thaugsuban, who led the protest against Yingluck Shinawatra's Pheu Thai Party government from 2013 to 2014 that paved the way for the 2014 coup, expressed his strong support to the draft constitution and called it an anti-corruption one and said it will help to reform the country.
Abhisit Vejjajiva, former prime minister and current leader of Democrat Party, said the draft constitution has big defects in combating corruption.
Pheu Thai Party said the draft exempts the NCPO from any punishments, which is not fair.
On the other hand, Abhisit also said the draft cannot help to ease political and social conflict but trigger even more serious conflict, primarily due to the reduced roles of the elected politicians.
According to reports, former premier Thaksin Shinawatra on Thursday commented on the draft, saying "The drafters ... created a constitution for the 'continuity' of the absolute power of the present coup makers to continue even after the new constitution is proclaimed."
Thailand held a referendum on a draft constitution in 2007 after the 2006 military coup that toppled Thaksin's government.
The draft then passed by 57 per cent approval and later became the 2007 constitution, under which Thaksin's sister Yingluck Shinawatra and her Pheu Thai party won the general election in 2011.
But her government was again toppled by a military coup in 2014 and the 2007 constitution was repealed and replaced by the 2014 interim constitution.