Foreign Minister Don Pramudwinai has accused foreign envoys of breaching diplomatic protocol and intervening in the justice system by being present when Future Forward Party leader Thanathorn Juangroongruangkit reported to Pathumwan police on a sedition charge.
"That could not happen in their own countries, but they did it in our country. We will ask them to cooperate and not to do that again. It was against the diplomatic protocols of the United Nations," Mr Don said at Government House on Tuesday.
"As it has happened, there must be discussions on principles and etiquette, to create understanding. It is a basic conduct and embassies must know it. The (Foreign) Ministry once sent a letter and summoned ambassadors to tell them that such an incident was not appropriate."
Asked if foreign diplomats could attend if they were invited, Mr Don said that in principle such an invitation was unacceptable because it concerned the justice system.
"What happened is unusual. It is an intervention in our justice system," the foreign minister said.
"They think that they can do whatever they want in our country. We must ask Thai people whether it is appropriate for anyone to do anything when the feelings of Thai people are involved."
Mr Don said the Foreign Ministry was considering whether to summon the diplomats concerned to discuss the issue.
On Saturday, diplomats from Australia, Belgium, Canada, Finland, France, Germany, the Netherlands, the UK and the US, as well as EU and UN human rights officials, went to the police station to observe the proceedings.
Mr Thanathorn was charged with sedition under Section 116 of the Criminal Code, helping a suspect escape under Section 189, and organising an assembly of more than 10 people that caused unrest (Section 215). If convicted, he could face respective jail terms of seven years, two years and six months for the three charges.