An arch-royalist Thai monk led a protest Friday at the United States Embassy as acrimony deepened following the American envoy's criticism of the use of the kingdom's tough royal defamation law.
Thailand's once water-tight relationship with the US has come under strain since the 2014 coup, with Bangkok cosying up to Beijing in response to western censure of the military power grab.
On Friday afternoon around 200 people led by firebrand monk Luang Pu Buddha Issara gathered outside the US Embassy in downtown Bangkok, defying a ban on demonstrations by Thailand's junta.
A few dozen police officers stood in front of the embassy as protesters held placards saying "This is Thailand, not the USA" and "Go, Go Home" accusing the recently appointed US ambassador of meddling in the country's domestic affairs.
Their ire has been piqued by comments by Glyn T. Davies on Wednesday registering US concern over "the lengthy and unprecedented prison sentences handed down by Thai military courts against civilians" for breaching the lese majeste law.
Thailand's King Bhumibol Adulyadej, 87, is perceived as a near-deity by many in the country.
He is protected by one of the world's strictest royal defamation laws, prosecutions under which have surged since the military seized power.
Issara, a nationalist Buddhist monk who also led protests against the toppled former civilian government, read a statement to the crowd condemning Davies and "pressure to make us change" the royal slur law.
"You have no right and no power... we are not slaves of the US. The monarchy is a sacred symbol that all Thais are ready to defend with their lives," he added.
With the diplomatic spat intensifying, Thai premier Prayut Chan-O-Cha also waded into the debate on Friday.
"It's up to him (Davies). Next time don't send anyone to talk about trade with me then," Prayut told reporters on Friday when asked about the US envoy's comments.
Prayut, a former army chief, is infamous for his gruff responses to critical questions.
The army pitches itself as the defender of the monarchy and says it is duty-bound to crack down on those who insult the institution.
But critics say the lese majeste law is used as a political tool against opponents of the coup.
US officials have urged a swift return to democracy and an end to repression - particularly the increasingly tough sentences for royal defamation.
Under the law anyone convicted of insulting the king, queen, heir or regent can face up to 15 years in jail on each count.
The military has stepped up its patrol of alleged lese majeste offences, especially on social media, and recent convictions have seen record terms of 30 and 28 years handed to two Thais for committing royal defamation on Facebook.
Local and international media based in Thailand routinely self-censor when reporting on the royals for fear of falling foul of the law.
The spike in prosecutions comes with the reign of the ailing king entering its twilight.