On the eve of this election, Thailand's King Maha Vajiralongkorn - invoking a decades-old speech made by his now-deceased father - issued a surprise statement calling for Thais to support "good people" and "control the bad people" to prevent them ruling the country.

On Sunday morning, as voting got under way after 8, the phrase "already grown up, can choose for myself" was trending on social media across Thailand - reflecting, if nothing else, Thais' eagerness to have their say in who runs their country.

Despite Thais' enthusiasm for the democratic vote - and while most seat-by-seat results will be known within hours of the polls closing on Sunday evening  - weeks of uncertainty lay ahead as parties horse trade and attempt to form a coalition government before the official declaration of results on May 9.

That uncertainty stems, in part, from a skewed electoral system in the country's new constitution that hands a big advantage to incumbent military junta leader Prayut Chan-o-cha.

Under the complicated new system there are 350 constituency seats and150 party list seats in the 500 member lower house.

But the Senate is comprised of 250 members, all of whom are hand-picked by the military junta - and expected to back Prayut - and the next prime minister will be chosen at a joint sitting, needing a majority of 376 votes from the 750 MPs in the house and senate.

That could in effect mean Prayut has a 250 vote head start and that he and his allies will need to win just 126 of the 500 lower house seats to be chosen as prime minister.

Sunday's poll broadly pits Prayut against the popular Pheu Thai party, which remains loyal to exiled former prime minister Thaksin Shinawatra.

Shinawatra and his allies, sometimes known as Red Shirts, have won every democratic election since 2001, but both he in 2006, and his sister Yingluck in 2014, were removed from power in military coups.

Those coups were supported by the so-called Yellow Shirts and whistle blowers - a group that includes sections of Bangkok's influential middle class and royalists - who alleged corruption from the Thaksin camp.

A number of other parties including the Democrats, led by former prime minister Abhisit Vejjajiva, and the new Future Forward Party, led by charismatic 40-year-old billionaire Thanathorn Juangroongruangkit, are expected to perform well too.

University student Thammachart Chatapalaboon, 22, voted for Future Forward Party on Sunday as he was bored with “old politics”, including military-backed parties.

"
Actually, anyone but Prayut is fine for me. I have had enough of him. To be fair, most of the projects he has done that he usually promotes every Friday [on his national TV program] aren’t bad but I prefer a leader who is more mature," he said outside a Bangkok polling station.

Architect Ake Patanaichai, 53, said he had voted for the Democrats as he believed they were the least evil of the major political parties.

His dream team would be Prayut Chan-o-cha as prime minister and Korn Chatikavanij (a senior Democrat) as economic minister.

“I’d like to vote for Prayut but the party that supports him has bad guys that I can't accept. I voted Democrat as I personally admired Korn and their policies are most practical and are likely to be implemented immediately," he said.

Political analyst and former editor of Bangkok's The Nation newspaper Tulsathit Taptim said there were four post-election scenarios that could be considered most likely to play out - and three would see Prayut re-installed as the nation's leader.

The first was a substantial win for Prayut and his Palang Pracharat Party camp, which would see Prayut easily installed as prime minister.

The second was an indecisive result in which neither side secured a majority, so the hand-picked 250 member senate helped installed Prayut as prime minister - with a period of marked instability to follow.

The third scenario, a variation on the second, would again see Prayut installed as leader but would be followed by small and medium-sized parties defecting to support behind the former general - allowing a period of stability in Thai politics.

Only in the fourth scenario would the opposition parties, including Pheu Thai, Future Forward and other smaller offshoot parties, emerge to claim the prime ministership and end the junta's rule - and this was dependent on huge swings to the opposition parties.