A key red-shirt supporter in Phetchabun province was arrested in the early hours of yesterday over suspicions of widely disseminating a false announcement that was purportedly issued by the Royal Household Bureau.
The suspect is now being detained and interrogated by police and Army officers at the Eleventh Infantry Division in Bangkok.
He can be held for seven days without charge under martial law.
Kris Buddeechin, 26, was arrested at 2am yesterday after a combined force of police and Army officers raided his house in Phetchabun's Muang district and found him using a notebook computer.
The officers also confiscated other computers and mobile phones, as well as hard disks and CDs carrying red-shirt propaganda songs. He was tracked down after police traced the fake document.
Two more suspects were searched, one of whom is a special branch police officer, but nothing was found.
Kris is a former deputy leader of the Phetchabun branch of the red-shirt United Front for Democracy against Dictatorship.
Police spokesperson Pol Lt-General Prawut Thavornsiri said the man was still being interrogated as of press time yesterday and that he will face a lese majeste charge as well as one on violating the Computer Crimes Act.
Prawut said police monitored how many times the forged document was shared via Facebook, Twitter and Line from 9pm on Monday night and found that most of the "shares" originated from Phetchabun province, which eventually lead to Kris's arrest.
The nom de plume used by him on Facebook and Twitter was Nest Oishi.
Police chief Pol General Somyos Pumpanmuang said the suspect has confessed to sharing the document on social media, adding that he had some 4,000-5,000 "friends" on Facebook and that he wanted them to know about the announcement.
Police believe the mastermind behind this fake document lives overseas.
Prime Minister General Prayut Chan-o-cha had earlier ordered a manhunt for the person behind the fake document, eventually leading to the arrest of Kris, who is a musician by profession.