BANGKOK - Thai citizens began voting at 8am on Sunday (9am Singapore time) in the first nationwide gauge of public sentiment since the military staged a coup in 2014.

They will decide on the fate of a draft Constitution which its authors promise will clamp down on corruption and weed out political excesses, but which critics decry as a way to cement military power without the need for future coups.

While there has been some debate about health and education entitlements for citizens under this draft Charter, analysts expect voters to largely use the referendum to voice their approval or rejection of military rule.

If the draft charter passes this referendum, general elections will be held next year (2017).

Prime Minister Prayut Chan-o-cha, who on Friday declared that he would vote "yes", has insisted the election will go head next year no matter what happens in the referendum, though he has not stated how it will be done.

Election officials spent Saturday trying to fix last-minute hitches to an online results reporting system that they hoped would allow the votes from some 93,300 polling stations to be collated some four hours after voting ends at 4pm Thailand time.

An alcohol ban kicked in at 6pm on Saturday and will last until midnight Sunday.

Meanwhile, in the southern border provinces where militants are waging a separatist insurgency, several small explosions occurred on Friday night. The authorities described these as attempts to intimidate voters.

Two human rights activists were arrested in Chaiyaphum province on Saturday for allegedly distributing "vote no" material, reported local media.

Even though strict referendum rules have tampered down public debate about the draft Charter, Thailand's election commission expects some 80 per cent of the 50.6 million eligible voters to show up.

The 105-page document gives the ruling junta the power to pick almost all 250 senators during the transitory five-year period. Six seats in the Upper House will be reserved for senior security officials. While the future prime minister need not come from an election, the future elected government will have to abide by a "national strategy plan", or risk being impeached.

A second question on the ballot paper will ask the voter if he agrees with having senators join the elected House of Representatives in choosing a prime minister - in effect opening the possibility for the military to install a non-elected premier of its choice.

Former prime Yingluck Shinawatra, who was ousted by a Constitutional Court ruling shortly before her caretaker government was deposed by the 2014 coup, has declared that she would reject the Constitution.

Similarly, Mr Abhisit Vejjajiva, who heads the Democrat Party that helped to topple Ms Yingluck's Puea Thai government in 2014, has announced he would vote no. In his view, the Charter fails to resolve the Kingdom's deep set political conflicts and might even create new ones through the lack of proper representation.