BANGKOK — A Thai military court Friday sentenced a 67-year-old man to 18 months in prison for writing insults against the country's monarch on the wall of a shopping mall restroom, a lawyer said.

The military court in Bangkok found Opas Charnsooksai guilty of lese majeste — the crime of defaming, insulting or threatening the monarchy, for writing graffiti on the restroom wall in a Bangkok mall last October, the defendant's lawyer Sasinan Thamnithinan said.

Thailand's lese majeste law is the harshest in the world, providing for jail terms of three to 15 years. After invoking martial law and staging a May 22 coup, the military has intensified a crackdown on criticism of the monarchy and announced that prosecution of all such cases will be in military courts.

The court cut the original 3-year prison sentence in half because Opas confessed, but said in passing sentence that it would not suspend the jail term because "the offense was violating the beloved monarch of the people," Sasinan, who works for the Thai Lawyers for Human Rights Center, told The Associated Press.

Opas has been detained since he was arrested in October and repeated bail requests have been rejected by the military court.

King Bhumibol Adulyadej, 87, the world's longest-reigning monarch, is in poor health. His fading from public life and the palace's perceived role in political battles that began in 2006 have tarnished the institution in recent years, undermining what had previously been near-universal respect for the royal institution.