Polling stations in Thailand opened on Sunday morning for general elections to choose members of the country's lower house of parliament and prime minister.

Around 90,000 polling stations across the country opened at 8am local time, and will close at 5pm.

Fifty-one million people are eligible to vote, including seven million young Thais who will be participating in their first ever election.

Sunday will be the first time Thais have the chance to vote since a bloodless coup in 2014, led by current prime minister General Prayuth Chan-ocha, overthrew the elected government of Yingluck Shinawatra, the sister of former prime minister Thaksin.

Both are living in self-imposed exile after being found guilty in absentia of corruption and sentenced to five year prison terms.

The Shinawatras, who enjoy huge support from Thailand's poor and rural classes, particularly in the north and northeast of the country, have won every poll held in the Southeast Asian nation since 2001.

Sunday's elections are the first since a new constitution enacted following the death of revered King Bhumibol Adulyadej in 2016 banned large parties and ensured that the military oversees a 20-strategic plan, regardless of which party wins.

The constitution, approved with the promise of providing stability to the country and preventing a potential stalemate in parliament, also gives the military establishment powers to nominate all 250 members of the senate (the upper house) for a five-year term.

Voters on Sunday will elect 500 members of the House of Representatives - the lower house - for a four-year term.

All 750 representatives from the two houses will vote together to elect the next prime minister.

Bhumibol's son and successor, King Maha Vajiralongkorn, made a surprise statement late on Saturday night, the eve of the polls, urging Thais to elect "good people" to rule and prevent "bad people" from "creating chaos".

That announcement came just two months after a royal decree barred his eldest sister, Princess Ubolratana, from running as a candidate for prime minister.

The Thai Raksa Chart party, which is linked to the Shinawatras, had nominated the hugely popular and respected princess in an attempt to swing the vote against parties backed by the ruling military junta.

But after the King called the move "highly inappropriate", the party was dissolved by the electoral commission in a major blow to the Shinawatra clan.

Thaksin Shinawatra's enormous popularity is largely thanks to his approval of universal healthcare and soft loans for farmers in 2001 to alleviate inequality, which earned him massive support in rural areas and among the working classes.

Pitted against him were the wealthiest classes and the elites close to the monarchy and the army, who ousted him in 2006 and toppled the Yingluck government in 2014.

Pre-election surveys indicate that the Shinawatra-backed Pueu Thai, which was overthrown in 2014, will secure most of the votes on Sunday, while pro-military parties, such as Prayut's Palang Pracharat, are not expected to fare well.

The Democrat Party, the country's oldest, which enjoys support among the middle classes and in the south, and the newly-formed Anakot Mai (Future Forward), which is hugely popular among younger voters, are also expected to win significant numbers of seats in parliament.