The election will determine the makeup of Thailand's parliament, which has 500 elective seats. Those elected MPs and 250 unelected senators, appointed by the junta, will decide who becomes prime minister. It is unclear how the raw number for votes will translate into seats under a complicated new system of MPs elected through constituency races and proportional representation.
The prime minister is chosen by a simple majority.
As polls opened yesterday in Thailand, a hashtag started trending on social media: #OldEnoughtoVoteOurselves.
Throughout the day, Thailand's millennials were among the most enthusiastic voters, rushing to reject the dominance of the military junta. They argued with elder relatives, shared political videos on social media and subtly challenged Thailand's social contract, where the word of the monarchy is absolute, with hashtags.
"We want to see new things from new people, rather than the same old politicians talking about the same things," said a 32-year old who wanted to be named only by her nickname, Kob, for fear of repercussion from her government-linked employer. "We want a prime minister that comes from an election, not a coup."
Of the 52 million eligible voters in Thailand - almost 70 per cent of whom turned up to cast ballots in the first election since a 2014 coup - about seven million are young, first-time voters.
The leader of Future Forward Party, Thanathorn Juangroongruangkit, has been cited with energising youths who grew up under military rule, creating a dedicated fan club that mobs the fresh-faced billionaire and clamours for selfies.
The Washington Post interviewed more than a dozen voters under 35, and most said they picked Thanathorn's party, while their parents and grandparents voted for the army party.
The generational split mirrors trends across the world, including in the United States and Britain, where the young have backed liberal causes while their elders have leaned conservative.
The hashtag appeared to be a response to an unusual statement from Thai King Vajiralongkorn ahead of the vote, in which he encouraged voters to pick "good people" as their leaders and stop "bad people" from obtaining power and causing turmoil.
Some voters, speaking on the condition of partial anonymity because criticism of the Thai monarchy is a crime punishable by jail time, rejected the paternalistic undertones of the message and said they wanted to make their own choices. Others have explicitly rejected taking political cues from their elders.