From Hmong hill people voting in the shadow of mist-cloaked jungle mountains to Spider-Man cosplayers on Bangkok's scorching streets, millions turned out to cast their ballots in Thailand's election on Sunday.

Voters are tipped to deliver a resounding defeat to incumbent Prime Minister Prayut Chan-o-Cha after a campaign that has played out as a clash between a reformist young generation and the conservative, military-backed establishment.

Some 52 million people were registered to vote across the sprawling kingdom. 

In northern Chiang Mai, hundreds of Hmong hill people queued to vote in an echoing teak-pillared hall as roosters crowed, shortly after polls opened at 8am (0100 GMT).

"This election is good for everybody since we will see a big change that we have been waiting for the past eight years," Jidapa Wangwanapat told AFP.

"If this election turns out well and there's no corruption, then I think the country would be better," the 22-year-old said.

Opposition parties have led in polling ahead of the election, building on voters' dissatisfaction with a struggling economy and a feeble post-pandemic recovery.

"I expect this election will change Thailand for the better since the economy is really bad at the moment," Maethawee Wangwanapat told AFP.

"I hope this election will move the country into a better direction," said Maethawee, 30, wearing a traditional Hmong blue velvet embroidered jacket.

Hmong people - who in Thailand mainly live in northern Chiang Mai and Chiang Rai - have a complicated history with central Thailand and some continue to face discrimination.

'This is their future'

Hundreds of miles south near the ancient capital Ayutthaya, elephants helped at a local polling station, local media reported, with photos of the pachyderms queueing with voters.

The scenes in the capital Bangkok were no less colourful as two men dressed as Spider-Man, one carrying a guitar, entertained people on the hot streets.

And while most prime ministerial candidates took a quiet luxury approach to their outfits, Bhumjaithai party leader Anutin Charnvirakul - who oversaw the legalisation of cannabis in Thailand - voted wearing a shirt decorated with the green leaf.

On the other end of the spectrum, pro-democracy activist Parit Chiwarak, who goes by the nickname "Penguin", voted with a cardboard box on his head.

Emblazoned with the slogan "EC don't cheat the election", his message was a reference to Thailand's Electoral Commission and fears that, in a country with a long history of coups, the result of the ballot may not be respected.

This election was vital, 85-year-old Pakorn Adulpan told AFP, explaining he had voted in every election during his lifetime.

"Every Thai citizen should come out to vote, especially the young ones," he said, after voting in a wheelchair.

"This is their future," he said.