A Cambodian opposition activist facing rare charges of insulting the king was released on Tuesday after he issued a public apology to the monarch and long-ruling leader Hun Sen.

Two opposition figures -- Yim Sinorn and Hun Kosal -- were arrested last week and charged with insulting the king and incitement to commit serious unrest over posts on Facebook.

The pair wrote the posts after the king, Prime Minister Hun Sen and senior government officials attended a televised ceremony at Angkor Wat temple to light the torch for the upcoming Southeast Asia Games in Cambodia.

Sinorn, a close aide to opposition leader Kem Sokha, wrote "according to the voice of people from coffee shops, today we can clearly see who is the real King".

But on Tuesday evening he issued a public apology to King Norodom Sihamoni and Hun Sen, asking for forgiveness for his "unintentional mistake".

A judge then ordered prison officials "to release the accused Yim Sinorn" following a decision to free him on bail, according to a warrant seen by AFP.

Human rights activists had previously warned enactment of the 2018 lese majeste law, which carries a sentence of between one and five years in prison, could be wielded to target dissent.

His critics say Hun Sen, one of the world's longest-ruling leaders, has wound back democratic freedoms since he came to power in 1985, using the courts to stifle opposition.

Earlier this month, Kem Sokha was sentenced to 27 years in jail and placed under house arrest for treason over an alleged plot with foreigners to topple Hun Sen's government.

Cambodians will go to polls for a general election in July.

AFP