Thai police on Sunday charged a woman with royal defamation after a mob demanded action over a Facebook post allegedly smearing the "heir and regent", as the country mourns King Bhumibol Adulyadej.

Thailand has one of the world's harshest lese majeste laws, with jail terms of up to 15 years for each count of defaming or insulting the king, queen, heir or regent.

The woman, who has not been named, was accused of posting a derogatory statement on Facebook on Friday, according to Thewes Pleumsud of Bo Pud police in the southeastern island of Koh Samui.

"She did not post against the late King -- it involved the heir and the regent," he said, referring to Crown Prince Maha Vajiralongkorn, and the 96-year-old Prem Tinsulanonda who in a surprise move became temporary regent on Friday.

He declined to give further details since doing so could violate the catch-all law.

Prem, a former prime minister and Bhumibol's Privy Council head, will act as regent until the Crown Prince formally ascends the throne.

An angry mob descended on Bo Pud police station on Sunday demanding the woman be charged.

The crowd hurled insults at the woman, according to videos widely shared on Facebook.

Police said she was charged and then publicly prostrated herself in apology before a portrait of the king, who died on Thursday aged 88 -- prompting a wave of grief across the nation.

Two other similar cases since the king's death -- in which angry crowds urged punishment for alleged royal defamation on social media -- have raised fears of mob action.

Domestic and foreign media outlets based in the country routinely self-censor to avoid falling foul of the broadly worded law, while social opprobrium follows those perceived to have overstepped the mark.

Critics say the law -- known as '112' after its criminal code -- has encouraged witch hunts by the public, with police and courts obliged to investigate all accusations.

The law prevents all but the most cursory public discussion of Thailand's monarchy, or reporting or debate on the issue.

Cases have surged since royalist generals ousted a civilian government from power in 2014.

The generals have vowed to defend the monarchy from criticism. But analysts say the law has overwhelmingly been used to skewer their political rivals.

A number of people with mental health problems have also been prosecuted.

Cyber patrol teams trawl the internet for royal insults, while self-appointed ultra-royalist civilian groups also monitor the web and report violations.