Two Cambodian opposition figures have been charged under the country's rarely used lese majeste law with insulting the king for posts made on social media about a photograph of the king and the prime minister.

A judge at the Phnom Penh Municipal Court also charged Yim Sinorn and Hun Kosal with incitement to cause serious social unrest in the country, a court document showed.

The men, once members of the now-dissolved opposition Cambodia National Rescue Party (CNRP), could face a jail term of up to five years and fines of up to $US2,500 ($A3,718) if found guilty.

They were arrested on Tuesday over their Facebook posts about a photograph of King Norodom Sihamoni and Prime Minister Hun Sen standing together at a torch relay ceremony for the upcoming South East Asian Games.

The court did not specify what the posts said.

Neither Yim Sinorn or Hun Kosal nor their legal representatives could immediately be reached for comment.

Yim Sinorn later posted on Facebook that he had deleted what he wrote about the king, and Hun Kosal said he respected the king and would promote the royal family.

Prime Minister Hun Sen in comments on his official Facebook page about the arrests and before the men were charged said: "This is an insulting act that cannot be tolerated or excused."

He said the men should never be forgiven, while denying that his remarks were aimed at pressuring the court.

Cambodia's lese majeste law was unanimously adopted by parliament in 2018. Rights groups expressed concerns at the time that the law, which is similar to legislation in neighbouring Thailand, could be used to silence government critics.

The CNRP was banned ahead of a 2018 election that was swept by Hun Sen's Cambodian People's Party (CPP).

CNRP has since been decimated, with many of its members arrested or fleeing into exile in what activists say is a sweeping crackdown designed to thwart challenges to the CPP's power monopoly.

Kem Sokha, who once headed CNRP, was sentenced on March 3 to 27 years of house arrest after being found guilty of treason, in a case condemned by the United States as politically motivated.

He had denied the charges he was conspiring with the United States to overthrow Hun Sen, who has ruled Cambodia for nearly four decades.

Reuters