Dozens of demonstrators gathered at the United States embassy in Bangkok on Friday to protest against new American ambassador Glyn Davies' criticism of Thailand's lese majeste law.
Led by activist abbot Buddha Isara, they urged Mr Davies, who took up his post just nine weeks ago, to better understand Thai culture and not to intervene in the debates related to politics and the royal institution.
They called on the ambassador to consider the longstanding good ties between both countries. Demonstrators raised banners in Thai and English expressing their disapproval of his comments.
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 envoy of meddling in the country's domestic affairs.
"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," Abbot Buddha Isara said.
Abbot Buddha Isara presented a letter to communicate the protesters' message to an embassy official. He said Mr Davies had strongly criticised the law that protected the monarchy, which was very disrespectful behaviour for an ambassador.
Abbot Buddha Isara: "We are not slaves of the US"
Punishments for offences against the royal institution were justified by the behaviours of wrongdoers, the abbot said.
With the diplomatic spat intensifying, Prime Minister Prayut Chan-o-Cha also waded into the debate on Friday.
"It's up to him," he said, referring to Mr Davies. "Next time don't send anyone to talk about trade with me then," Gen Prayut told reporters when asked about the US envoy's comments.
The United States has been lobbying Thailand in recent weeks to consider joining the 12-country Trans-Pacific partnership, which already includes four other Asean nations.
On Wednesday Mr Davies expressed concern about prison terms handed down under Section 112 of the Criminal Code, the lese majeste law.
No one should be jailed for peacefully expressing their opinion, he said during a talk at the Foreign Correspondents' Club of Thailand.
"We're also concerned by the lengthy and unprecedented prison sentences handed down by Thai military courts against civilians for violating the lese majeste law," Mr Davies said.
Under the lese majeste law, a conviction for insulting the King, Queen, heir or regent faces up to 15 years in prison.