The United Nations Human Rights Office for South-East Asia (OHCHR) has urged the government to stop the practice of arbitrary detention of political activists, and to immediately release people recently charged with criticising authorities.
They were referring to the detention of human rights lawyer Prawet Prapanukul and Danai Tibsuya, 34, a beer company employee, along with four other individuals.
"I am concerned at the sharp increase in the use of the lese-majeste law after the 2014 coup, with more than 70 people detained or convicted," said Laurent Meillan, the acting Regional Representative.
He added the implementation of the law runs contrary to the fundamental rights of freedom of expression and opinion.
- Earlier report: Lawyer faces 150 years in prison
Mr Prawet faces consecutive sentences of up to 150 years in prison if convicted on 10 counts of lese majeste, Thai Lawyers for Human Rights (TLHR) said.
Mr Prawet, 57, has in the past provided legal assistance to members of the United Front for Democracy against Dictatorship and served as legal counsel for lese majeste suspect Daranee Charnchoengsilpakul, better known as Da Torpedo. He was detained in a raid on his home by police and troops last weekend.
Mr Prawet appeared in Bangkok Criminal Court Thursday, charged with 10 counts of lese majeste -- Section 112 of the Criminal Code -- and three counts of breaching Section 116, also known as the sedition law.
"Prawet faces 10 counts of breaking Section 112, so that works out to up to 150 years in prison if he is found guilty," said Anon Nampha of TLHR, a legal watchdog.
"He also faces three separate charges of breaking Section 116 (for incitement under the Criminal Code), which carries a maximum sentence of seven years in prison for each offence." A spokesman for the military government said the authorities had no comment on the case.
The lawyers' group said Mr Prawet was charged along with five others, who also faced charges under the lese majeste law.
He is in pre-trial detention at Bangkok remand prison after being held at the 11th Army Circle base in Bangkok, a facility the military uses as a temporary prison.
The court granted a police request to further detain all six until May 14, it was reported.
The identities of the other four were not released.
Mr Prawet is accused of posting 10 messages insulting the monarch and three messages with content believed to instigate social disorder, police told the court. He denied the claims.
Mr Danai denied similar accusations, but further details were not available.
The TLHR said this raised questions about the police force's request to detain Mr Danai.
Three of the others allegedly admitted sharing messages by Thammasat University history professor Somsak Jeamteerasakul on their Facebook pages. The messages referred to the controversial disappearance of a plaque commemorating the 1932 Siamese Revolution from the Royal Plaza last month.
The sixth person facing the charges has reportedly denied all of the accusations.