Thais are being urged by both major political parties and the ruling government to go to polls to decide on a junta-drafted constitution, the first democratic vote in the country since the military took power in May 2014.
Leaders from both parties and the junta were on hand to cast their ballot in front of press and supporters before noon (1500 AEST) on Sunday. They urged the rest of the country to do the same.
"I urge Thais to come out through rain, the more rain there is the more you should come vote," said Prime Minister Prayut Chan-o-cha.
"It is an historic decision, so let's make this a big turnout."
"If you do not come to vote, you are not taking advantage of your democratic rights," said former premier Yingluck Shinawatra, whose government was ousted by the military.
Local media reported several hiccups as Thais went to the polls. In Laksi district in Northern Bangkok, several people were unable to find their name on voting lists.
In Southern Thailand, security was tightened after several overnight bomb attacks targeted phone lines and electricity polls.
Authorities say that the events were unlikely linked to the referendum and were more likely part of a long-running separatist insurgency.
At one polling booth in Nonthaburi, 20 kilometres north of Bangkok, at least 20 people had arrived before the 8am opening time to cast their ballot.
"I think it's important to be part of the democratic process as we have had no say for over two years. It's important for everyday citizens to have their voices heard," said Chanya Nitikorn, a 42-year-old housewife.
Chanya and others, however, refused to discuss how they would vote citing several laws passed by the government which forbid public discussion of the charter.
A cadre of student activists, politicians and journalists have been arrested under the law for voicing disapproval of the proposed constitution.
Both main political parties have come out against a yes vote.
Detractors argue that the charter would prolong military rule by allowing for a fully appointed upper house of parliament and provisions for an unelected Prime Minister.