BANGKOK (AFP) - A 67-year-old Thai man has been charged with insulting the monarchy after scribbling anti-royal comments on public toilet walls, an official said Friday, the latest charge under the controversial lese majeste law.

Thailand's revered King Bhumibol Adulyadej, 86, is protected by tough royal defamation rules under which anyone convicted of insulting the king, queen, heir or regent faces up to 15 years in prison on each count.

Opas Charnsooksai was arrested by the military in a shopping mall in eastern Bangkok Wednesday for writing anti-monarchy messages in the public toilets, army officer Burin Thongprapai told AFP.

"Security guards at the mall discovered he had written insulting comments on walls in the restroom... He confessed to the charge of lese majeste," he said, adding they had obtained CCTV evidence against Opas.

The accused will now be tried in a military court - with no right to appeal - under martial law, which was imposed by the army two days before it seized power in May's coup.

Burin added that a raid on Opas' house revealed "symbols" of the opposition Red Shirt movement.

Last month rights group Amnesty International said there had been an "unprecedented" number of people charged with insulting the Thai monarchy since the coup.

In August a 28-year-old musician was sentenced to 15 years in jail for writing insulting Facebook posts about the monarchy between 2010 and 2011.

In another recent case a taxi driver was jailed for two and a half years after his passenger - a university lecturer who recorded their conversation on a mobile phone - accused him of expressing anti-royal views, Amnesty said.

Since seizing power Thailand's junta chief Prayuth Chan-ocha, who in August was also appointed as premier, has made several warnings that his regime will not tolerate any criticism of the royal family.

Critics say the legislation has been politicised, noting that many of those charged were linked to the Red Shirts, who are broadly supportive of fugitive former premier Thaksin Shinawatra.

Thaksin, the older brother of ousted prime minister Yingluck Shinawatra, was toppled in a previous coup in 2006 and lives in self-exile to avoid prison for a corruption conviction.

Thailand's long-running political conflict broadly pits a Bangkok-based middle class and royalist elite, backed by parts of the military and judiciary, against rural and working-class voters loyal to Thaksin.