Cambodia made insulting the king a crime in February, triggering alarm from rights groups who warned the law would likely become another government tool to target dissent.
The Southeast Asian country has a constitutional monarch, King Norodom Sihamoni, but is tightly run by its authoritarian premier Hun Sen, whose government relies on an arsenal of laws to silence foes.
Police said Kheang Navy, a 50-year-old principal, was detained in central Cambodia's Kampong Thom province on Saturday over the Facebook post.
"We arrested him for writing comments that insulted the King," said deputy provincial police chief Yeng Sareth, declining to elaborate on the post's content.
"For lese majeste, this is the first case," he added.
In neighbouring Thailand, which is known for its frequent and brutal deployment of a draconian lese majeste law, repeating any alleged royal insults can be grounds for prosecution.
A court spokesman said Kheang Navy was charged with the offence, which carries up to five years in prison, on Sunday and then sent to prison for pre-trial detention.
The case comes as Cambodia kicks off a three-day celebration marking the King's 65th birthday on Monday.
King Sihamoni, who took the throne in 2004, is considered a purely symbolic head of state who is largely above the political fray.
The power of Cambodia's monarchy has declined significantly in recent decades under Hun Sen, a domineering leader who has amassed strong sway over the country's courts, security forces and economy during his 33 years in office.
In recent months, the strongman's government has waged a sweeping crackdown on political rivals, critical NGOs and independent media in the run-up to July 2018 elections.
Cambodia was showered with criticism from rights groups and western democracies last year after the Supreme Court ruled to disband the main opposition party -- a verdict that effectively ensures a Hun Sen victory at the ballot box.
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