A Cambodian elder statesman leaked a call meant to sooth a border spat between the two nations in which Paetongtarn called him "uncle" and referred to a Thai military commander as her "opponent".

A key party abandoned Paetongtarn's coalition, accusing the 38-year-old dynastic premier of cow-towing to Cambodia and undermining Thailand's military, leaving her teetering with a slim parliamentary majority.

Around 4,000 demonstrators filled roads ringing the capital's Victory Monument, waving Thai flags and cheering for speeches interspersed with live music.

The crowd was mostly senior-aged and led by veteran activists of the "Yellow Shirt" movement, which helped oust Paetongtarn's father Thaksin in the 2000s, as well as one of his former allies now among his harshest critics.

"I'm here to protect Thailand's sovereignty and to say the PM is unfit," said 70-year-old protester Seri Sawangmue, who travelled overnight by bus from the country's north to attend.

"After I heard the leaked call, I knew I couldn't trust her," he told AFP. "I've lived through many political crises and I know where this is going. She's willing to give up our sovereignty."

Thailand has seen decades of clashes between the bitterly-opposed "Yellow Shirts" who defend the monarchy and military and the "Red Shirts" backing Thaksin, who they consider a threat to Thailand's traditional social order.

Jamnong Kalana, 64, said she was once a "Red Shirt" but had now changed her colours and was demanding Paetongtarn's resignation.

"I feel full of pain when I see a fellow Thai who doesn't love the country like I do," she said.

Authorities said more than 1,000 police and 100 city officials had been deployed for the event which remained peaceful early on Saturday afternoon.

Paetongtarn was visiting Thailand's flood-hit north but before departing Bangkok she told reporters: "It's their right to protest, as long as it's peaceful".

The prime minister has been battered by controversy and abandoned by her largest backer the Bhumjaithai Party after her phone call with Cambodia's ex-leader Hun Sen leaked earlier this month.

Tensions between the countries have soared after a border dispute boiled over into violence last month which killed one Cambodia soldier.

Next week, both Paetongtarn and her father face legal battles that could reshape Thailand's political landscape.

On Tuesday, the Constitutional Court will decide whether to take up a petition by senators seeking her removal over alleged unprofessionalism. That same day, her father is set to stand trial on royal defamation charges linked to decade-old remarks to South Korean media.

Paetongtarn took office less than a year ago after her predecessor was disqualified by a court order and her father returned from exile after 15 years.

She is the fourth Shinawatra-linked figure to become prime minister following her father, aunt and uncle-in-law.