The kingdom's politics have been dominated for years by a battle between the conservative, pro-military, pro-royalist elite and the Shinawatra clan, who they consider a threat to Thailand's traditional social order.

Thaksin Shinawatra, the 75-year-old family patriarch and billionaire twice elected leader in the early 2000s, has arrived at a Bangkok criminal court to face accusations of breaching strict lese majeste laws used to shield Thailand's king from criticism.

The allegations stem from a 2015 interview he gave to South Korean media and he faces up to 15 years in jail after the trial set to last for weeks, with a verdict not expected for at least a month after that.

A court official confirmed his arrival to AFP on Tuesday morning and said the trial had started but media would not be allowed in.

"I can't speak on his behalf about how he feels, but I think he seems chill," his lawyer Winyat Chatmontri told AFP outside court.

Meanwhile, Thailand's Constitutional Court is due to meet for the first time since a group of conservative senators lodged a case against Paetongtarn, accusing her of breaching ministerial ethics during a diplomatic spat with Cambodia.

If the court decides to hear the case, they could suspend her as they enter months-long deliberations, plunging Thailand into chaos as it grapples with a spluttering economy and the threat of US tariffs.

"I will let the process take its course," a downcast Paetongtarn told reporters on Monday. "If you are asking whether I am worried, I am."

If Paetongtarn is suspended, power would pass to her deputy Phumtham Wechayachai.

The 38-year-old Paetongtarn took office less than a year ago but has been badly weakened by a scandal over her conduct in the row with neighboring Cambodia.

Phone call scandal

A long-standing territorial dispute boiled over into cross-border clashes in May, killing one Cambodian soldier.

When Paetongtarn called Cambodian ex-leader Hun Sen to discuss the tensions, she called him "uncle" and referred to a Thai military commander as her "opponent", according to a leaked recording which caused widespread backlash.

Conservative lawmakers accused her of kowtowing to Cambodia and undermining the military, and alleged she breached constitutional provisions requiring "evident integrity" and "ethical standards" among ministers.

Paetongtarn's Pheu Thai party has already been abandoned by a key conservative coalition partner, leaving her with a razor-thin parliamentary majority dependent on other parties.

Thailand's king on Tuesday approved Paetongtarn's cabinet reshuffle after her allies quit. 

She assigned herself the culture minister position and is due to take up the position on Thursday, but it is unclear if she could take up that role while under investigation by the Constitutional Court.

Around 10,000 people mustered in central Bangkok over the weekend to protest her administration.

Her approval rating has plunged to just nine percent, down from around 30 percent three months ago, according to a survey released Sunday by Bangkok university the National Institute of Development Administration.

Paetongtarn's case and her father's trial are the latest round in a bitter, decades-long tussle between Thailand's powerful conservative forces, and parties linked to Thaksin.

Thaksin was ousted in a coup in 2006, while his sister Yingluck Shinawatra suffered the same fate in 2014 and other prime ministers from their political movement have been sacked by court rulings.

After 15 years abroad, Thaksin returned to Thailand in August 2023.

He was immediately ordered to serve an eight-year jail term for historic graft and abuse of power charges, but was taken to hospital on health grounds and later pardoned by the king.