The case is the largest known lese majeste prosecution so far under Thailand's new King Maha Vajiralongkorn, who ascended the throne after the October death of his father.
Observers have been closely watching how the law -- which bars any criticism of the royal family and forces media to heavily self-censor -- is applied under his reign.
The five, who have not been named, were remanded in custody Wednesday for sharing Facebook posts written by a Paris-based Thai historian Somsak Jeamteerasakul about the disappearance of a democracy plaque in Bangkok last month.
"They shared Somsak's posts about the 1932 plaque," Anon Numpa, from Thai Lawyers for Human Rights, told AFP without elaborating on what the posts said.
The small bronze monument, which lay in Bangkok's heavily-policed Royal Plaza, marked the 1932 revolution that ended absolute monarchy in Thailand.
It was mysteriously replaced by a new plaque bearing a royalist message in April, sparking fury among Thailand's hard-pressed democracy campaigners who fear the monument was removed with official backing.
Authorities have moved to stifle discourse on the missing plaque and have detained several activists who have tried to call for an investigation.
Thai authorities last month warned that contacting, following or sharing social media posts by junta critic Somsak could be grounds for legal action.
The five faced charges alongside prominent human rights lawyer Prawet Prapanukul, who was hit with a record ten counts of royal defamation for a series of social media posts apparently containing republican sentiment.
Prawet now faces a maximum 150-year sentence under a law that assigns up to 15 years in prison per offence.
A police officer from the technology crime suppression division confirmed the men were remanded over lese majeste charges but declined to comment further.
Royal insult cases are typically shrouded in secrecy as even repeating defamatory content can be grounds for prosecution.
In a recent Facebook post, 57-year-old Prawet encouraged Thais to push the boundaries of the lese majeste law, which has choked expression across the kingdom's media, academia and arts.
Use of the law has shot up under the royalist junta that seized power in 2014.
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