An international human rights group says use of Thailand's harsh law against criticizing the monarchy has intensified under military rule.

A report issued Friday by the Paris-based International Federation for Human Rights says prosecutions under the lese majeste law since the Thai army ousted an elected government in May 2014 have infringed on the rights to liberty, a fair trial and freedom of expression and violate international agreements endorsed by Thailand.

It says six people were in detention under the law at the time of the military coup, but the number has now increased nearly nine-fold to 53.

Defaming the king, queen or heir-apparent is punishable by three to 15 years' imprisonment per incident. The military says the law is necessary to safeguard the monarchy and national security.